USB Gadet API for Linux

USB Gadget API for Linux

David Brownell

<dbrownell@users.sourceforge.net>

Copyright © 2003-2004 David Brownell

This documentation is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA

For more details see the file COPYING in the source distribution of Linux.

Table of Contents

1. Introduction2. Structure of Gadget Drivers3. Kernel Mode Gadget API
Driver Life CycleUSB 2.0 Chapter 9 Types and ConstantsCore Objects and MethodsOptional UtilitiesComposite Device FrameworkComposite Device Functions4. Peripheral Controller Drivers5. Gadget Drivers6. USB On-The-GO (OTG)

Chapter 1. Introduction

This document presents a Linux-USB "Gadget" kernel mode API, for use within peripherals and other USB devices that embed Linux. It provides an overview of the API structure, and shows how that fits into a system development project. This is the first such
API released on Linux to address a number of important problems, including:

Supports USB 2.0, for high speed devices which can stream data at several dozen megabytes per second.

Handles devices with dozens of endpoints just as well as ones with just two fixed-function ones. Gadget drivers can be written so they're easy to port to new hardware.

Flexible enough to expose more complex USB device capabilities such as multiple configurations, multiple interfaces, composite devices, and alternate interface settings.

USB "On-The-Go" (OTG) support, in conjunction with updates to the Linux-USB host side.

Sharing data structures and API models with the Linux-USB host side API. This helps the OTG support, and looks forward to more-symmetric frameworks (where the same I/O model is used by both host and device side drivers).

Minimalist, so it's easier to support new device controller hardware. I/O processing doesn't imply large demands for memory or CPU resources.

Most Linux developers will not be able to use this API, since they have USB "host" hardware in a PC, workstation, or server. Linux users with embedded systems are more likely to have USB peripheral hardware. To distinguish drivers running inside such hardware
from the more familiar Linux "USB device drivers", which are host side proxies for the real USB devices, a different term is used: the drivers inside the peripherals are "USB gadget drivers". In USB protocol interactions, the device driver is the master (or
"client driver") and the gadget driver is the slave (or "function driver").

The gadget API resembles the host side Linux-USB API in that both use queues of request objects to package I/O buffers, and those requests may be submitted or canceled. They share common definitions for the standard USB Chapter
9 messages, structures, and constants. Also, both APIs bind and unbind drivers to devices. The APIs differ in detail, since the host side's current URB framework exposes a number of implementation details and assumptions that are inappropriate
for a gadget API. While the model for control transfers and configuration management is necessarily different (one side is a hardware-neutral master, the other is a hardware-aware slave), the endpoint I/0 API used here should also be usable for an overhead-reduced
host side API.

Chapter 2. Structure of Gadget Drivers

A system running inside a USB peripheral normally has at least three layers inside the kernel to handle USB protocol processing, and may have additional layers in user space code. The "gadget" API is used by the middle layer to interact with the lowest level
(which directly handles hardware).

In Linux, from the bottom up, these layers are:

USB Controller Driver
This is the lowest software level. It is the only layer that talks to hardware, through registers, fifos, dma, irqs, and the like. The 

<linux/usb/gadget.h>

API abstracts the peripheral controller endpoint hardware. That hardware
is exposed through endpoint objects, which accept streams of IN/OUT buffers, and through callbacks that interact with gadget drivers. Since normal USB devices only have one upstream port, they only have one of these drivers. The controller driver can support
any number of different gadget drivers, but only one of them can be used at a time.

Examples of such controller hardware include the PCI-based NetChip 2280 USB 2.0 high speed controller, the SA-11x0 or PXA-25x UDC (found within many PDAs), and a variety of other products.

Gadget Driver
The lower boundary of this driver implements hardware-neutral USB functions, using calls to the controller driver. Because such hardware varies widely in capabilities and restrictions, and is used in embedded environments where space is at a premium, the
gadget driver is often configured at compile time to work with endpoints supported by one particular controller. Gadget drivers may be portable to several different controllers, using conditional compilation. (Recent kernels substantially simplify the work
involved in supporting new hardware, by autoconfiguring endpoints automatically for many bulk-oriented drivers.) Gadget driver responsibilities include:

handling setup requests (ep0 protocol responses) possibly including class-specific functionality

returning configuration and string descriptors

(re)setting configurations and interface altsettings, including enabling and configuring endpoints

handling life cycle events, such as managing bindings to hardware, USB suspend/resume, remote wakeup, and disconnection from the USB host.

managing IN and OUT transfers on all currently enabled endpoints

Such drivers may be modules of proprietary code, although that approach is discouraged in the Linux community.

Upper Level
Most gadget drivers have an upper boundary that connects to some Linux driver or framework in Linux. Through that boundary flows the data which the gadget driver produces and/or consumes through protocol transfers over USB. Examples include:

user mode code, using generic (gadgetfs) or application specific files in 

/dev

networking subsystem (for network gadgets, like the CDC Ethernet Model gadget driver)

data capture drivers, perhaps video4Linux or a scanner driver; or test and measurement hardware.

input subsystem (for HID gadgets)

sound subsystem (for audio gadgets)

file system (for PTP gadgets)

block i/o subsystem (for usb-storage gadgets)

... and more

Additional Layers
Other layers may exist. These could include kernel layers, such as network protocol stacks, as well as user mode applications building on standard POSIX system call APIs such as open(), close(), read() and write().
On newer systems, POSIX Async I/O calls may be an option. Such user mode code will not necessarily be subject to the GNU General Public License (GPL).

OTG-capable systems will also need to include a standard Linux-USB host side stack, with usbcore, one or more Host Controller Drivers (HCDs), USB Device
Drivers to support the OTG "Targeted Peripheral List", and so forth. There will also be an OTG Controller Driver, which is visible to gadget and device driver developers only indirectly. That helps the host
and device side USB controllers implement the two new OTG protocols (HNP and SRP). Roles switch (host to peripheral, or vice versa) using HNP during USB suspend processing, and SRP can be viewed as a more battery-friendly kind of device wakeup protocol.

Over time, reusable utilities are evolving to help make some gadget driver tasks simpler. For example, building configuration descriptors from vectors of descriptors for the configurations interfaces and endpoints is now automated, and many drivers now use
autoconfiguration to choose hardware endpoints and initialize their descriptors. A potential example of particular interest is code implementing standard USB-IF protocols for HID, networking, storage, or audio classes. Some developers are interested in KDB
or KGDB hooks, to let target hardware be remotely debugged. Most such USB protocol code doesn't need to be hardware-specific, any more than network protocols like X11, HTTP, or NFS are. Such gadget-side interface drivers should eventually be combined, to implement
composite devices.

Chapter 3. Kernel Mode Gadget API

Table of Contents

Driver Life CycleUSB 2.0 Chapter 9 Types and ConstantsCore Objects and MethodsOptional UtilitiesComposite Device FrameworkComposite Device Functions
Gadget drivers declare themselves through a struct usb_gadget_driver, which is responsible for most parts of enumeration for a struct usb_gadget. The response to a set_configuration
usually involves enabling one or more of the struct usb_ep objects exposed by the gadget, and submitting one or more struct usb_request buffers to transfer data. Understand those
four data types, and their operations, and you will understand how this API works.

Incomplete Data Type Descriptions

This documentation was prepared using the standard Linux kernel 

docproc

 tool, which turns text and in-code comments into SGML DocBook and then into usable formats such as HTML or PDF. Other than the "Chapter 9" data types, most
of the significant data types and functions are described here.

However, docproc does not understand all the C constructs that are used, so some relevant information is likely omitted from what you are reading. One example of such information is endpoint autoconfiguration. You'll have to read the header file, and use
example source code (such as that for "Gadget Zero"), to fully understand the API.

The part of the API implementing some basic driver capabilities is specific to the version of the Linux kernel that's in use. The 2.6 kernel includes a driver model framework that has no analogue on earlier kernels;
so those parts of the gadget API are not fully portable. (They are implemented on 2.4 kernels, but in a different way.) The driver model state is another part of this API that is ignored by the kerneldoc tools.

The core API does not expose every possible hardware feature, only the most widely available ones. There are significant hardware features, such as device-to-device DMA (without temporary storage in a memory buffer) that would be added using hardware-specific
APIs.

This API allows drivers to use conditional compilation to handle endpoint capabilities of different hardware, but doesn't require that. Hardware tends to have arbitrary restrictions, relating to transfer types, addressing, packet sizes, buffering, and availability.
As a rule, such differences only matter for "endpoint zero" logic that handles device configuration and management. The API supports limited run-time detection of capabilities, through naming conventions for endpoints. Many drivers will be able to at least
partially autoconfigure themselves. In particular, driver init sections will often have endpoint autoconfiguration logic that scans the hardware's list of endpoints to find ones matching the driver requirements (relying on those conventions), to eliminate
some of the most common reasons for conditional compilation.

Like the Linux-USB host side API, this API exposes the "chunky" nature of USB messages: I/O requests are in terms of one or more "packets", and packet boundaries are visible to drivers. Compared to RS-232 serial protocols, USB resembles synchronous protocols
like HDLC (N bytes per frame, multipoint addressing, host as the primary station and devices as secondary stations) more than asynchronous ones (tty style: 8 data bits per frame, no parity, one stop bit). So for example the controller drivers won't buffer
two single byte writes into a single two-byte USB IN packet, although gadget drivers may do so when they implement protocols where packet boundaries (and "short packets") are not significant.

Driver Life Cycle

Gadget drivers make endpoint I/O requests to hardware without needing to know many details of the hardware, but driver setup/configuration code needs to handle some differences. Use the API like this:

Register a driver for the particular device side usb controller hardware, such as the net2280 on PCI (USB 2.0), sa11x0 or pxa25x as found in Linux PDAs, and so on. At this point the device is logically in the USB ch9 initial state ("attached"), drawing no
power and not usable (since it does not yet support enumeration). Any host should not see the device, since it's not activated the data line pullup used by the host to detect a device, even if VBUS power is available.

Register a gadget driver that implements some higher level device function. That will then bind() to a usb_gadget, which activates the data line pullup sometime after detecting VBUS.

The hardware driver can now start enumerating. The steps it handles are to accept USB power and set_address requests. Other steps are handled by the gadget driver. If the gadget driver module is unloaded before the host starts to enumerate, steps before
step 7 are skipped.

The gadget driver's setup() call returns usb descriptors, based both on what the bus interface hardware provides and on the functionality being implemented. That can involve alternate settings or configurations, unless the hardware prevents such operation.
For OTG devices, each configuration descriptor includes an OTG descriptor.

The gadget driver handles the last step of enumeration, when the USB host issues a set_configuration call. It enables all endpoints used in that configuration, with all interfaces in their default settings. That involves using a list of the hardware's endpoints,
enabling each endpoint according to its descriptor. It may also involve using 

usb_gadget_vbus_draw

 to let more power be drawn from VBUS, as allowed by that configuration. For OTG devices, setting a configuration may also involve
reporting HNP capabilities through a user interface.

Do real work and perform data transfers, possibly involving changes to interface settings or switching to new configurations, until the device is disconnect()ed from the host. Queue any number of transfer requests to each endpoint. It may be suspended and
resumed several times before being disconnected. On disconnect, the drivers go back to step 3 (above).

When the gadget driver module is being unloaded, the driver unbind() callback is issued. That lets the controller driver be unloaded.

Drivers will normally be arranged so that just loading the gadget driver module (or statically linking it into a Linux kernel) allows the peripheral device to be enumerated, but some drivers will defer enumeration until some higher level component (like
a user mode daemon) enables it. Note that at this lowest level there are no policies about how ep0 configuration logic is implemented, except that it should obey USB specifications. Such issues are in the domain of gadget drivers, including knowing about implementation
constraints imposed by some USB controllers or understanding that composite devices might happen to be built by integrating reusable components.

Note that the lifecycle above can be slightly different for OTG devices. Other than providing an additional OTG descriptor in each configuration, only the HNP-related differences are particularly visible to driver code. They involve reporting requirements
during the SET_CONFIGURATION request, and the option to invoke HNP during some suspend callbacks. Also, SRP changes the semantics of 

usb_gadget_wakeup

 slightly.

USB 2.0 Chapter 9 Types and Constants

Gadget drivers rely on common USB structures and constants defined in the 

<linux/usb/ch9.h>

 header file, which is standard in Linux 2.6 kernels. These are the same types and constants used by host side drivers (and usbcore).

Name

struct usb_ctrlrequest — SETUP data for a USB device control request

Synopsis

struct usb_ctrlrequest {
__u8 bRequestType;
__u8 bRequest;
__le16 wValue;
__le16 wIndex;
__le16 wLength;
};

Members

bRequestType
matches the USB bmRequestType field

bRequest
matches the USB bRequest field

wValue
matches the USB wValue field (le16 byte order)

wIndex
matches the USB wIndex field (le16 byte order)

wLength
matches the USB wLength field (le16 byte order)

Description

This structure is used to send control requests to a USB device. It matches the different fields of the USB 2.0 Spec section 9.3, table 9-2. See the USB spec for a fuller description of the different fields, and what they are used for.

Note that the driver for any interface can issue control requests. For most devices, interfaces don't coordinate with each other, so such requests may be made at any time.

Name

usb_endpoint_num — get the endpoint's number

Synopsis

int usb_endpoint_num (

epd ) ;

const struct usb_endpoint_descriptor * epd

;
 

Arguments

epd

endpoint to be checked

Description

Returns 

epd

's number: 0 to 15.

Name

usb_endpoint_type — get the endpoint's transfer type

Synopsis

int usb_endpoint_type (

epd ) ;

const struct usb_endpoint_descriptor * epd

;
 

Arguments

epd

endpoint to be checked

Description

Returns one of USB_ENDPOINT_XFER_{CONTROL, ISOC, BULK, INT} according to 

epd

's transfer type.

Name

usb_endpoint_dir_in — check if the endpoint has IN direction

Synopsis

int usb_endpoint_dir_in (

epd ) ;

const struct usb_endpoint_descriptor * epd

;
 

Arguments

epd

endpoint to be checked

Description

Returns true if the endpoint is of type IN, otherwise it returns false.

Name

usb_endpoint_dir_out — check if the endpoint has OUT direction

Synopsis

int usb_endpoint_dir_out (

epd ) ;

const struct usb_endpoint_descriptor * epd

;
 

Arguments

epd

endpoint to be checked

Description

Returns true if the endpoint is of type OUT, otherwise it returns false.

Name

usb_endpoint_xfer_bulk — check if the endpoint has bulk transfer type

Synopsis

int usb_endpoint_xfer_bulk (

epd ) ;

const struct usb_endpoint_descriptor * epd

;
 

Arguments

epd

endpoint to be checked

Description

Returns true if the endpoint is of type bulk, otherwise it returns false.

Name

usb_endpoint_xfer_control — check if the endpoint has control transfer type

Synopsis

int usb_endpoint_xfer_control (

epd ) ;

const struct usb_endpoint_descriptor * epd

;
 

Arguments

epd

endpoint to be checked

Description

Returns true if the endpoint is of type control, otherwise it returns false.

Name

usb_endpoint_xfer_int — check if the endpoint has interrupt transfer type

Synopsis

int usb_endpoint_xfer_int (

epd ) ;

const struct usb_endpoint_descriptor * epd

;
 

Arguments

epd

endpoint to be checked

Description

Returns true if the endpoint is of type interrupt, otherwise it returns false.

Name

usb_endpoint_xfer_isoc — check if the endpoint has isochronous transfer type

Synopsis

int usb_endpoint_xfer_isoc (

epd ) ;

const struct usb_endpoint_descriptor * epd

;
 

Arguments

epd

endpoint to be checked

Description

Returns true if the endpoint is of type isochronous, otherwise it returns false.

Name

usb_endpoint_is_bulk_in — check if the endpoint is bulk IN

Synopsis

int usb_endpoint_is_bulk_in (

epd ) ;

const struct usb_endpoint_descriptor * epd

;
 

Arguments

epd

endpoint to be checked

Description

Returns true if the endpoint has bulk transfer type and IN direction, otherwise it returns false.

Name

usb_endpoint_is_bulk_out — check if the endpoint is bulk OUT

Synopsis

int usb_endpoint_is_bulk_out (

epd ) ;

const struct usb_endpoint_descriptor * epd

;
 

Arguments

epd

endpoint to be checked

Description

Returns true if the endpoint has bulk transfer type and OUT direction, otherwise it returns false.

Name

usb_endpoint_is_int_in — check if the endpoint is interrupt IN

Synopsis

int usb_endpoint_is_int_in (

epd ) ;

const struct usb_endpoint_descriptor * epd

;
 

Arguments

epd

endpoint to be checked

Description

Returns true if the endpoint has interrupt transfer type and IN direction, otherwise it returns false.

Name

usb_endpoint_is_int_out — check if the endpoint is interrupt OUT

Synopsis

int usb_endpoint_is_int_out (

epd ) ;

const struct usb_endpoint_descriptor * epd

;
 

Arguments

epd

endpoint to be checked

Description

Returns true if the endpoint has interrupt transfer type and OUT direction, otherwise it returns false.

Name

usb_endpoint_is_isoc_in — check if the endpoint is isochronous IN

Synopsis

int usb_endpoint_is_isoc_in (

epd ) ;

const struct usb_endpoint_descriptor * epd

;
 

Arguments

epd

endpoint to be checked

Description

Returns true if the endpoint has isochronous transfer type and IN direction, otherwise it returns false.

Name

usb_endpoint_is_isoc_out — check if the endpoint is isochronous OUT

Synopsis

int usb_endpoint_is_isoc_out (

epd ) ;

const struct usb_endpoint_descriptor * epd

;
 

Arguments

epd

endpoint to be checked

Description

Returns true if the endpoint has isochronous transfer type and OUT direction, otherwise it returns false.

Core Objects and Methods

These are declared in 

<linux/usb/gadget.h>

, and are used by gadget drivers to interact with USB peripheral controller drivers.

Name

struct usb_request — describes one i/o request

Synopsis

struct usb_request {
void * buf;
unsigned length;
dma_addr_t dma;
unsigned no_interrupt:1;
unsigned zero:1;
unsigned short_not_ok:1;
void (* complete) (struct usb_ep *ep,struct usb_request *req);
void * context;
struct list_head list;
int status;
unsigned actual;
};

Members

buf
Buffer used for data. Always provide this; some controllers only use PIO, or don't use DMA for some endpoints.

length
Length of that data

dma
DMA address corresponding to 'buf'. If you don't set this field, and the usb controller needs one, it is responsible for mapping and unmapping the buffer.

no_interrupt
If true, hints that no completion irq is needed. Helpful sometimes with deep request queues that are handled directly by DMA controllers.

zero
If true, when writing data, makes the last packet be “short” by adding a zero length packet as needed;

short_not_ok
When reading data, makes short packets be treated as errors (queue stops advancing till cleanup).

complete
Function called when request completes, so this request and its buffer may be re-used. The function will always be called with interrupts disabled, and it must not sleep. Reads terminate with a short packet, or when the buffer fills, whichever comes first.
When writes terminate, some data bytes will usually still be in flight (often in a hardware fifo). Errors (for reads or writes) stop the queue from advancing until the completion function returns, so that any transfers invalidated by the error may first be
dequeued.

context
For use by the completion callback

list
For use by the gadget driver.

status
Reports completion code, zero or a negative errno. Normally, faults block the transfer queue from advancing until the completion callback returns. Code“-ESHUTDOWN” indicates completion caused by device
disconnect, or when the driver disabled the endpoint.

actual
Reports bytes transferred to/from the buffer. For reads (OUT transfers) this may be less than the requested length. If the short_not_ok flag is set, short reads are treated as errors even when status otherwise indicates successful completion. Note that for
writes (IN transfers) some data bytes may still reside in a device-side FIFO when the request is reported as complete.

Description

These are allocated/freed through the endpoint they're used with. The hardware's driver can add extra per-request data to the memory it returns, which often avoids separate memory allocations (potential failures), later when the request is queued.

Request flags affect request handling, such as whether a zero length packet is written (the “zero” flag), whether a short read should be treated as an error (blocking request queue advance, the “short_not_ok” flag),
or hinting that an interrupt is not required (the “no_interrupt” flag, for use with deep request queues).

Bulk endpoints can use any size buffers, and can also be used for interrupt transfers. interrupt-only endpoints can be much less functional.

NOTE

this is analogous to 'struct urb' on the host side, except that it's thinner and promotes more pre-allocation.

Name

struct usb_ep — device side representation of USB endpoint

Synopsis

struct usb_ep {
void * driver_data;
const char * name;
const struct usb_ep_ops * ops;
struct list_head ep_list;
unsigned maxpacket:16;
};

Members

driver_data
for use by the gadget driver. all other fields are read-only to gadget drivers.

name
identifier for the endpoint, such as “ep-a” or “ep9in-bulk”

ops
Function pointers used to access hardware-specific operations.

ep_list
the gadget's ep_list holds all of its endpoints

maxpacket
The maximum packet size used on this endpoint. The initial value can sometimes be reduced (hardware allowing), according to the endpoint descriptor used to configure the endpoint.

Description

the bus controller driver lists all the general purpose endpoints in gadget->ep_list. the control endpoint (gadget->ep0) is not in that list, and is accessed only in response to a driver 

setup

 callback.

Name

usb_ep_enable — configure endpoint, making it usable

Synopsis

int usb_ep_enable (	ep,
	desc ) ;

struct usb_ep * ep

;

const struct usb_endpoint_descriptor * desc

;
 

Arguments

ep

the endpoint being configured. may not be the endpoint named “ep0”. drivers discover endpoints through the ep_list of a usb_gadget.

desc

descriptor for desired behavior. caller guarantees this pointer remains valid until the endpoint is disabled; the data byte order is little-endian (usb-standard).

Description

when configurations are set, or when interface settings change, the driver will enable or disable the relevant endpoints. while it is enabled, an endpoint may be used for i/o until the driver receives a 

disconnect

 from the host
or until the endpoint is disabled.

the ep0 implementation (which calls this routine) must ensure that the hardware capabilities of each endpoint match the descriptor provided for it. for example, an endpoint named “ep2in-bulk” would be
usable for interrupt transfers as well as bulk, but it likely couldn't be used for iso transfers or for endpoint 14. some endpoints are fully configurable, with more generic names like “ep-a”. (remember
that for USB, “in” means “towards the USB master”.)

returns zero, or a negative error code.

Name

usb_ep_disable — endpoint is no longer usable

Synopsis

int usb_ep_disable (

ep ) ;

struct usb_ep * ep

;
 

Arguments

ep

the endpoint being unconfigured. may not be the endpoint named “ep0”.

Description

no other task may be using this endpoint when this is called. any pending and uncompleted requests will complete with status indicating disconnect (-ESHUTDOWN) before this call returns. gadget drivers must call 

usb_ep_enable

 again
before queueing requests to the endpoint.

returns zero, or a negative error code.

Name

usb_ep_alloc_request — allocate a request object to use with this endpoint

Synopsis

struct usb_request * usb_ep_alloc_request (	ep,
	gfp_flags ) ;

struct usb_ep * ep

;

gfp_t gfp_flags

;
 

Arguments

ep

the endpoint to be used with with the request

gfp_flags

GFP_* flags to use

Description

Request objects must be allocated with this call, since they normally need controller-specific setup and may even need endpoint-specific resources such as allocation of DMA descriptors. Requests may be submitted with 

usb_ep_queue

,
and receive a single completion callback. Free requests with 

usb_ep_free_request

, when they are no longer needed.

Returns the request, or null if one could not be allocated.

Name

usb_ep_free_request — frees a request object

Synopsis

void usb_ep_free_request (	ep,
	req ) ;

struct usb_ep * ep

;

struct usb_request * req

;
 

Arguments

ep

the endpoint associated with the request

req

the request being freed

Description

Reverses the effect of 

usb_ep_alloc_request

. Caller guarantees the request is not queued, and that it will no longer be requeued (or otherwise used).

Name

usb_ep_queue — queues (submits) an I/O request to an endpoint.

Synopsis

int usb_ep_queue (	ep,
	req,
	gfp_flags ) ;

struct usb_ep * ep

;

struct usb_request * req

;

gfp_t gfp_flags

;
 

Arguments

ep

the endpoint associated with the request

req

the request being submitted

gfp_flags

GFP_* flags to use in case the lower level driver couldn't pre-allocate all necessary memory with the request.

Description

This tells the device controller to perform the specified request through that endpoint (reading or writing a buffer). When the request completes, including being canceled by 

usb_ep_dequeue

, the request's completion routine
is called to return the request to the driver. Any endpoint (except control endpoints like ep0) may have more than one transfer request queued; they complete in FIFO order. Once a gadget driver submits a request, that request may not be examined or modified
until it is given back to that driver through the completion callback.

Each request is turned into one or more packets. The controller driver never merges adjacent requests into the same packet. OUT transfers will sometimes use data that's already buffered in the hardware. Drivers can rely on the fact that the first byte of
the request's buffer always corresponds to the first byte of some USB packet, for both IN and OUT transfers.

Bulk endpoints can queue any amount of data; the transfer is packetized automatically. The last packet will be short if the request doesn't fill it out completely. Zero length packets (ZLPs) should be avoided in portable protocols since not all usb hardware
can successfully handle zero length packets. (ZLPs may be explicitly written, and may be implicitly written if the request 'zero' flag is set.) Bulk endpoints may also be used for interrupt transfers; but the reverse is not true, and some endpoints won't support
every interrupt transfer. (Such as 768 byte packets.)

Interrupt-only endpoints are less functional than bulk endpoints, for example by not supporting queueing or not handling buffers that are larger than the endpoint's maxpacket size. They may also treat data toggle differently.

Control endpoints ... after getting a 

setup

 callback, the driver queues one response (even if it would be zero length). That enables the status ack, after transferring data as specified in the response. Setup functions may return
negative error codes to generate protocol stalls. (Note that some USB device controllers disallow protocol stall responses in some cases.) When control responses are deferred (the response is written after the setup callback returns), then 

usb_ep_set_halt

 may
be used on ep0 to trigger protocol stalls. Depending on the controller, it may not be possible to trigger a status-stage protocol stall when the data stage is over, that is, from within the response's completion routine.

For periodic endpoints, like interrupt or isochronous ones, the usb host arranges to poll once per interval, and the gadget driver usually will have queued some data to transfer at that time.

Returns zero, or a negative error code. Endpoints that are not enabled report errors; errors will also be reported when the usb peripheral is disconnected.

Name

usb_ep_dequeue — dequeues (cancels, unlinks) an I/O request from an endpoint

Synopsis

int usb_ep_dequeue (	ep,
	req ) ;

struct usb_ep * ep

;

struct usb_request * req

;
 

Arguments

ep

the endpoint associated with the request

req

the request being canceled

Description

if the request is still active on the endpoint, it is dequeued and its completion routine is called (with status -ECONNRESET); else a negative error code is returned.

note that some hardware can't clear out write fifos (to unlink the request at the head of the queue) except as part of disconnecting from usb. such restrictions prevent drivers from supporting configuration changes, even to configuration zero (a “chapter
9” requirement).

Name

usb_ep_set_halt — sets the endpoint halt feature.

Synopsis

int usb_ep_set_halt (

ep ) ;

struct usb_ep * ep

;
 

Arguments

ep

the non-isochronous endpoint being stalled

Description

Use this to stall an endpoint, perhaps as an error report. Except for control endpoints, the endpoint stays halted (will not stream any data) until the host clears this feature; drivers may need to empty the endpoint's request queue first, to make sure no
inappropriate transfers happen.

Note that while an endpoint CLEAR_FEATURE will be invisible to the gadget driver, a SET_INTERFACE will not be. To reset endpoints for the current altsetting, see 

usb_ep_clear_halt

. When switching altsettings, it's simplest to
use 

usb_ep_enable

 or 

usb_ep_disable

 for the endpoints.

Returns zero, or a negative error code. On success, this call sets underlying hardware state that blocks data transfers. Attempts to halt IN endpoints will fail (returning -EAGAIN) if any transfer requests are still queued, or if the controller hardware
(usually a FIFO) still holds bytes that the host hasn't collected.

Name

usb_ep_clear_halt — clears endpoint halt, and resets toggle

Synopsis

int usb_ep_clear_halt (

ep ) ;

struct usb_ep * ep

;
 

Arguments

ep

the bulk or interrupt endpoint being reset

Description

Use this when responding to the standard usb “set interface” request, for endpoints that aren't reconfigured, after clearing any other state in the endpoint's i/o queue.

Returns zero, or a negative error code. On success, this call clears the underlying hardware state reflecting endpoint halt and data toggle. Note that some hardware can't support this request (like pxa2xx_udc), and accordingly can't correctly implement interface
altsettings.

Name

usb_ep_set_wedge — sets the halt feature and ignores clear requests

Synopsis

int usb_ep_set_wedge (

ep ) ;

struct usb_ep * ep

;
 

Arguments

ep

the endpoint being wedged

Description

Use this to stall an endpoint and ignore CLEAR_FEATURE(HALT_ENDPOINT) requests. If the gadget driver clears the halt status, it will automatically unwedge the endpoint.

Returns zero on success, else negative errno.

Name

usb_ep_fifo_status — returns number of bytes in fifo, or error

Synopsis

int usb_ep_fifo_status (

ep ) ;

struct usb_ep * ep

;
 

Arguments

ep

the endpoint whose fifo status is being checked.

Description

FIFO endpoints may have “unclaimed data” in them in certain cases, such as after aborted transfers. Hosts may not have collected all the IN data written by the gadget driver (and reported by a request
completion). The gadget driver may not have collected all the data written OUT to it by the host. Drivers that need precise handling for fault reporting or recovery may need to use this call.

This returns the number of such bytes in the fifo, or a negative errno if the endpoint doesn't use a FIFO or doesn't support such precise handling.

Name

usb_ep_fifo_flush — flushes contents of a fifo

Synopsis

void usb_ep_fifo_flush (

ep ) ;

struct usb_ep * ep

;
 

Arguments

ep

the endpoint whose fifo is being flushed.

Description

This call may be used to flush the “unclaimed data” that may exist in an endpoint fifo after abnormal transaction terminations. The call must never be used except when endpoint is not being used for any
protocol translation.

Name

struct usb_gadget — represents a usb slave device

Synopsis

struct usb_gadget {
const struct usb_gadget_ops * ops;
struct usb_ep * ep0;
struct list_head ep_list;
enum usb_device_speed speed;
unsigned is_dualspeed:1;
unsigned is_otg:1;
unsigned is_a_peripheral:1;
unsigned b_hnp_enable:1;
unsigned a_hnp_support:1;
unsigned a_alt_hnp_support:1;
const char * name;
struct device dev;
};

Members

ops
Function pointers used to access hardware-specific operations.

ep0
Endpoint zero, used when reading or writing responses to driver 

setup

 requests

ep_list
List of other endpoints supported by the device.

speed
Speed of current connection to USB host.

is_dualspeed
True if the controller supports both high and full speed operation. If it does, the gadget driver must also support both.

is_otg
True if the USB device port uses a Mini-AB jack, so that the gadget driver must provide a USB OTG descriptor.

is_a_peripheral
False unless is_otg, the “A” end of a USB cable is in the Mini-AB jack, and HNP has been used to switch roles so that the “A” device currently acts
as A-Peripheral, not A-Host.

b_hnp_enable
OTG device feature flag, indicating that the A-Host enabled HNP support.

a_hnp_support
OTG device feature flag, indicating that the A-Host supports HNP at this port.

a_alt_hnp_support
OTG device feature flag, indicating that the A-Host only supports HNP on a different root port.

name
Identifies the controller hardware type. Used in diagnostics and sometimes configuration.

dev
Driver model state for this abstract device.

Description

Gadgets have a mostly-portable “gadget driver” implementing device functions, handling all usb configurations and interfaces. Gadget drivers talk to hardware-specific code indirectly, through ops vectors.
That insulates the gadget driver from hardware details, and packages the hardware endpoints through generic i/o queues. The “usb_gadget” and “usb_ep” interfaces
provide that insulation from the hardware.

Except for the driver data, all fields in this structure are read-only to the gadget driver. That driver data is part of the “driver model” infrastructure in 2.6 (and later) kernels, and for earlier systems
is grouped in a similar structure that's not known to the rest of the kernel.

Values of the three OTG device feature flags are updated before the 

setup

 call corresponding to USB_REQ_SET_CONFIGURATION, and before driver 

suspend

 calls. They are valid only when is_otg, and when
the device is acting as a B-Peripheral (so is_a_peripheral is false).

Name

gadget_is_dualspeed — return true iff the hardware handles high speed

Synopsis

int gadget_is_dualspeed (

g ) ;

struct usb_gadget * g

;
 

Arguments

g

controller that might support both high and full speeds

Name

gadget_is_otg — return true iff the hardware is OTG-ready

Synopsis

int gadget_is_otg (

g ) ;

struct usb_gadget * g

;
 

Arguments

g

controller that might have a Mini-AB connector

Description

This is a runtime test, since kernels with a USB-OTG stack sometimes run on boards which only have a Mini-B (or Mini-A) connector.

Name

usb_gadget_frame_number — returns the current frame number

Synopsis

int usb_gadget_frame_number (

gadget ) ;

struct usb_gadget * gadget

;
 

Arguments

gadget

controller that reports the frame number

Description

Returns the usb frame number, normally eleven bits from a SOF packet, or negative errno if this device doesn't support this capability.

Name

usb_gadget_wakeup — tries to wake up the host connected to this gadget

Synopsis

int usb_gadget_wakeup (

gadget ) ;

struct usb_gadget * gadget

;
 

Arguments

gadget

controller used to wake up the host

Description

Returns zero on success, else negative error code if the hardware doesn't support such attempts, or its support has not been enabled by the usb host. Drivers must return device descriptors that report their ability to support this, or hosts won't enable
it.

This may also try to use SRP to wake the host and start enumeration, even if OTG isn't otherwise in use. OTG devices may also start remote wakeup even when hosts don't explicitly enable it.

Name

usb_gadget_set_selfpowered — sets the device selfpowered feature.

Synopsis

int usb_gadget_set_selfpowered (

gadget ) ;

struct usb_gadget * gadget

;
 

Arguments

gadget

the device being declared as self-powered

Description

this affects the device status reported by the hardware driver to reflect that it now has a local power supply.

returns zero on success, else negative errno.

Name

usb_gadget_clear_selfpowered — clear the device selfpowered feature.

Synopsis

int usb_gadget_clear_selfpowered (

gadget ) ;

struct usb_gadget * gadget

;
 

Arguments

gadget

the device being declared as bus-powered

Description

this affects the device status reported by the hardware driver. some hardware may not support bus-powered operation, in which case this feature's value can never change.

returns zero on success, else negative errno.

Name

usb_gadget_vbus_connect — Notify controller that VBUS is powered

Synopsis

int usb_gadget_vbus_connect (

gadget ) ;

struct usb_gadget * gadget

;
 

Arguments

gadget

The device which now has VBUS power.

Context

can sleep

Description

This call is used by a driver for an external transceiver (or GPIO) that detects a VBUS power session starting. Common responses include resuming the controller, activating the D+ (or D-) pullup to let the host detect that a USB device is attached, and starting
to draw power (8mA or possibly more, especially after SET_CONFIGURATION).

Returns zero on success, else negative errno.

Name

usb_gadget_vbus_draw — constrain controller's VBUS power usage

Synopsis

int usb_gadget_vbus_draw (	gadget,
	mA ) ;

struct usb_gadget * gadget

;

unsigned mA

;
 

Arguments

gadget

The device whose VBUS usage is being described

mA

How much current to draw, in milliAmperes. This should be twice the value listed in the configuration descriptor bMaxPower field.

Description

This call is used by gadget drivers during SET_CONFIGURATION calls, reporting how much power the device may consume. For example, this could affect how quickly batteries are recharged.

Returns zero on success, else negative errno.

Name

usb_gadget_vbus_disconnect — notify controller about VBUS session end

Synopsis

int usb_gadget_vbus_disconnect (

gadget ) ;

struct usb_gadget * gadget

;
 

Arguments

gadget

the device whose VBUS supply is being described

Context

can sleep

Description

This call is used by a driver for an external transceiver (or GPIO) that detects a VBUS power session ending. Common responses include reversing everything done in 

usb_gadget_vbus_connect

.

Returns zero on success, else negative errno.

Name

usb_gadget_connect — software-controlled connect to USB host

Synopsis

int usb_gadget_connect (

gadget ) ;

struct usb_gadget * gadget

;
 

Arguments

gadget

the peripheral being connected

Description

Enables the D+ (or potentially D-) pullup. The host will start enumerating this gadget when the pullup is active and a VBUS session is active (the link is powered). This pullup is always enabled unless 

usb_gadget_disconnect

 has
been used to disable it.

Returns zero on success, else negative errno.

Name

usb_gadget_disconnect — software-controlled disconnect from USB host

Synopsis

int usb_gadget_disconnect (

gadget ) ;

struct usb_gadget * gadget

;
 

Arguments

gadget

the peripheral being disconnected

Description

Disables the D+ (or potentially D-) pullup, which the host may see as a disconnect (when a VBUS session is active). Not all systems support software pullup controls.

This routine may be used during the gadget driver 

bind

 call to prevent the peripheral from ever being visible to the USB host, unless later 

usb_gadget_connect

 is called. For example, user mode components
may need to be activated before the system can talk to hosts.

Returns zero on success, else negative errno.

Name

struct usb_gadget_driver — driver for usb 'slave' devices

Synopsis

struct usb_gadget_driver {
char * function;
enum usb_device_speed speed;
void (* unbind) (struct usb_gadget *);
int (* setup) (struct usb_gadget *,const struct usb_ctrlrequest *);
void (* disconnect) (struct usb_gadget *);
void (* suspend) (struct usb_gadget *);
void (* resume) (struct usb_gadget *);
struct device_driver driver;
};

Members

function
String describing the gadget's function

speed
Highest speed the driver handles.

unbind
Invoked when the driver is unbound from a gadget, usually from rmmod (after a disconnect is reported). Called in a context that permits sleeping.

setup
Invoked for ep0 control requests that aren't handled by the hardware level driver. Most calls must be handled by the gadget driver, including descriptor and configuration management. The 16 bit members of the setup data are in USB byte order. Called in_interrupt;
this may not sleep. Driver queues a response to ep0, or returns negative to stall.

disconnect
Invoked after all transfers have been stopped, when the host is disconnected. May be called in_interrupt; this may not sleep. Some devices can't detect disconnect, so this might not be called except as part of controller shutdown.

suspend
Invoked on USB suspend. May be called in_interrupt.

resume
Invoked on USB resume. May be called in_interrupt.

driver
Driver model state for this driver.

Description

Devices are disabled till a gadget driver successfully 

bind

s, which means the driver will handle 

setup

 requests needed to enumerate (and meet “chapter 9”requirements)
then do some useful work.

If gadget->is_otg is true, the gadget driver must provide an OTG descriptor during enumeration, or else fail the 

bind

 call. In such cases, no USB traffic may flow until both 

bind

 returns without
having called 

usb_gadget_disconnect

, and the USB host stack has initialized.

Drivers use hardware-specific knowledge to configure the usb hardware. endpoint addressing is only one of several hardware characteristics that are in descriptors the ep0 implementation returns from 

setup

 calls.

Except for ep0 implementation, most driver code shouldn't need change to run on top of different usb controllers. It'll use endpoints set up by that ep0 implementation.

The usb controller driver handles a few standard usb requests. Those include set_address, and feature flags for devices, interfaces, and endpoints (the get_status, set_feature, and clear_feature requests).

Accordingly, the driver's 

setup

 callback must always implement all get_descriptor requests, returning at least a device descriptor and a configuration descriptor. Drivers must make sure the endpoint descriptors match any hardware
constraints. Some hardware also constrains other descriptors. (The pxa250 allows only configurations 1, 2, or 3).

The driver's 

setup

 callback must also implement set_configuration, and should also implement set_interface, get_configuration, and get_interface. Setting a configuration (or interface) is where endpoints should be activated
or (config 0) shut down.

(Note that only the default control endpoint is supported. Neither hosts nor devices generally support control traffic except to ep0.)

Most devices will ignore USB suspend/resume operations, and so will not provide those callbacks. However, some may need to change modes when the host is not longer directing those activities. For example, local controls (buttons, dials, etc) may need to
be re-enabled since the (remote) host can't do that any longer; or an error state might be cleared, to make the device behave identically whether or not power is maintained.

Name

usb_gadget_probe_driver — probe a gadget driver

Synopsis

int usb_gadget_probe_driver (	driver,
	bind ) ;

struct usb_gadget_driver * driver

;

int (*bind) 
(
struct usb_gadget *[code])

;
 

Arguments

driver

the driver being registered

bind

the driver's bind callback

Context

can sleep

Description

Call this in your gadget driver's module initialization function, to tell the underlying usb controller driver about your driver. The 

bind

() function will be called to bind it to a gadget
before this registration call returns. It's expected that the 

bind

() function will be in init sections.

Name

usb_gadget_unregister_driver — unregister a gadget driver

Synopsis

int usb_gadget_unregister_driver (

driver ) ;

struct usb_gadget_driver * driver

;
 

Arguments

driver

the driver being unregistered

Context

can sleep

Description

Call this in your gadget driver's module cleanup function, to tell the underlying usb controller that your driver is going away. If the controller is connected to a USB host, it will first 

disconnect

. The driver is also requested
to 

unbind

 and clean up any device state, before this procedure finally returns. It's expected that the 

unbind

 functions will in in exit sections, so may not be linked in some kernels.

Name

struct usb_string — wraps a C string and its USB id

Synopsis

struct usb_string {
u8 id;
const char * s;
};

Members

id
the (nonzero) ID for this string

s
the string, in UTF-8 encoding

Description

If you're using 

usb_gadget_get_string

, use this to wrap a string together with its ID.

Name

struct usb_gadget_strings — a set of USB strings in a given language

Synopsis

struct usb_gadget_strings {
u16 language;
struct usb_string * strings;
};

Members

language
identifies the strings' language (0x0409 for en-us)

strings
array of strings with their ids

Description

If you're using 

usb_gadget_get_string

, use this to wrap all the strings for a given language.

Name

usb_free_descriptors — free descriptors returned by 

usb_copy_descriptors

Synopsis

void usb_free_descriptors (

v ) ;

struct usb_descriptor_header ** v

;
 

Arguments

v

vector of descriptors

Optional Utilities

The core API is sufficient for writing a USB Gadget Driver, but some optional utilities are provided to simplify common tasks. These utilities include endpoint autoconfiguration.

Name

usb_gadget_get_string — fill out a string descriptor

Synopsis

int usb_gadget_get_string (	table,
	id,
	buf ) ;

struct usb_gadget_strings * table

;

int id

;

u8 * buf

;
 

Arguments

table

of c strings encoded using UTF-8

id

string id, from low byte of wValue in get string descriptor

buf

at least 256 bytes

Description

Finds the UTF-8 string matching the ID, and converts it into a string descriptor in utf16-le. Returns length of descriptor (always even) or negative errno

If your driver needs stings in multiple languages, you'll probably “switch (wIndex) { ... }” in your ep0 string descriptor logic, using this routine after choosing which set of UTF-8 strings to use. Note
that US-ASCII is a strict subset of UTF-8; any string bytes with the eighth bit set will be multibyte UTF-8 characters, not ISO-8859/1 characters (which are also widely used in C strings).

Name

usb_descriptor_fillbuf — fill buffer with descriptors

Synopsis

int usb_descriptor_fillbuf (	buf,
	buflen,
	src ) ;

void * buf

;

unsigned buflen

;

const struct usb_descriptor_header ** src

;
 

Arguments

buf

Buffer to be filled

buflen

Size of buf

src

Array of descriptor pointers, terminated by null pointer.

Description

Copies descriptors into the buffer, returning the length or a negative error code if they can't all be copied. Useful when assembling descriptors for an associated set of interfaces used as part of configuring a composite device; or in other cases where
sets of descriptors need to be marshaled.

Name

usb_gadget_config_buf — builts a complete configuration descriptor

Synopsis

int usb_gadget_config_buf (	config,
	buf,
	length,
	desc ) ;

const struct usb_config_descriptor * config

;

void * buf

;

unsigned length

;

const struct usb_descriptor_header ** desc

;
 

Arguments

config

Header for the descriptor, including characteristics such as power requirements and number of interfaces.

buf

Buffer for the resulting configuration descriptor.

length

Length of buffer. If this is not big enough to hold the entire configuration descriptor, an error code will be returned.

desc

Null-terminated vector of pointers to the descriptors (interface, endpoint, etc) defining all functions in this device configuration.

Description

This copies descriptors into the response buffer, building a descriptor for that configuration. It returns the buffer length or a negative status code. The config.wTotalLength field is set to match the length of the result, but other descriptor fields (including
power usage and interface count) must be set by the caller.

Gadget drivers could use this when constructing a config descriptor in response to USB_REQ_GET_DESCRIPTOR. They will need to patch the resulting bDescriptorType value if USB_DT_OTHER_SPEED_CONFIG is needed.

Name

usb_copy_descriptors — copy a vector of USB descriptors

Synopsis

struct usb_descriptor_header ** usb_copy_descriptors (

src ) ;

struct usb_descriptor_header ** src

;
 

Arguments

src

null-terminated vector to copy

Context

initialization code, which may sleep

Description

This makes a copy of a vector of USB descriptors. Its primary use is to support usb_function objects which can have multiple copies, each needing different descriptors. Functions may have static tables of descriptors, which are used as templates and customized
with identifiers (for interfaces, strings, endpoints, and more) as needed by a given function instance.

Name

usb_find_endpoint — find a copy of an endpoint descriptor

Synopsis

struct usb_endpoint_descriptor * usb_find_endpoint (	src,
	copy,
	match ) ;

struct usb_descriptor_header ** src

;

struct usb_descriptor_header ** copy

;

struct usb_endpoint_descriptor * match

;
 

Arguments

src

original vector of descriptors

copy

copy of 

src

match

endpoint descriptor found in 

src

Description

This returns the copy of the 

match

 descriptor made for 

copy

. Its intended use is to help remembering the endpoint
descriptor to use when enabling a given endpoint.

Composite Device Framework

The core API is sufficient for writing drivers for composite USB devices (with more than one function in a given configuration), and also multi-configuration devices (also more than one function, but not necessarily sharing a given configuration). There
is however an optional framework which makes it easier to reuse and combine functions.

Devices using this framework provide a struct usb_composite_driver, which in turn provides one or more struct usb_configuration instances. Each such configuration includes at least
one struct usb_function, which packages a user visible role such as "network link" or "mass storage device". Management functions may also exist, such as "Device Firmware Upgrade".

Name

struct usb_function — describes one function of a configuration

Synopsis

struct usb_function {
const char * name;
struct usb_gadget_strings ** strings;
struct usb_descriptor_header ** descriptors;
struct usb_descriptor_header ** hs_descriptors;
struct usb_configuration * config;
int (* bind) (struct usb_configuration *,struct usb_function *);
void (* unbind) (struct usb_configuration *,struct usb_function *);
int (* set_alt) (struct usb_function *,unsigned interface, unsigned alt);
int (* get_alt) (struct usb_function *,unsigned interface);
void (* disable) (struct usb_function *);
int (* setup) (struct usb_function *,const struct usb_ctrlrequest *);
void (* suspend) (struct usb_function *);
void (* resume) (struct usb_function *);
};

Members

name
For diagnostics, identifies the function.

strings
tables of strings, keyed by identifiers assigned during 

bind

 and by language IDs provided in control requests

descriptors
Table of full (or low) speed descriptors, using interface and string identifiers assigned during 

bind

(). If this pointer is null, the function will not be available at full speed (or at
low speed).

hs_descriptors
Table of high speed descriptors, using interface and string identifiers assigned during 

bind

(). If this pointer is null, the function will not be available at high speed.

config
assigned when 

usb_add_function

() is called; this is the configuration with which this function is associated.

bind
Before the gadget can register, all of its functions 

bind

 to the available resources including string and interface identifiers used in interface or class descriptors; endpoints; I/O buffers; and so on.

unbind
Reverses 

bind

; called as a side effect of unregistering the driver which added this function.

set_alt
(REQUIRED) Reconfigures altsettings; function drivers may initialize usb_ep.driver data at this time (when it is used). Note that setting an interface to its current altsetting resets interface state, and that all interfaces have a disabled state.

get_alt
Returns the active altsetting. If this is not provided, then only altsetting zero is supported.

disable
(REQUIRED) Indicates the function should be disabled. Reasons include host resetting or reconfiguring the gadget, and disconnection.

setup
Used for interface-specific control requests.

suspend
Notifies functions when the host stops sending USB traffic.

resume
Notifies functions when the host restarts USB traffic.

Description

A single USB function uses one or more interfaces, and should in most cases support operation at both full and high speeds. Each function is associated by

usb_add_function

() with a one
configuration; that function causes 

bind

() to be called so resources can be allocated as part of setting up a gadget driver. Those resources include endpoints, which should be allocated
using 

usb_ep_autoconfig

().

To support dual speed operation, a function driver provides descriptors for both high and full speed operation. Except in rare cases that don't involve bulk endpoints, each speed needs different endpoint descriptors.

Function drivers choose their own strategies for managing instance data. The simplest strategy just declares it "static', which means the function can only be activated once. If the function needs to be exposed in more than one configuration at a given speed,
it needs to support multiple usb_function structures (one for each configuration).

A more complex strategy might encapsulate a 

usb_function

 structure inside a driver-specific instance structure to allows multiple activations. An example of multiple activations might
be a CDC ACM function that supports two or more distinct instances within the same configuration, providing several independent logical data links to a USB host.

Name

ep_choose — select descriptor endpoint at current device speed

Synopsis

struct usb_endpoint_descriptor * ep_choose (	g,
	hs,
	fs ) ;

struct usb_gadget * g

;

struct usb_endpoint_descriptor * hs

;

struct usb_endpoint_descriptor * fs

;
 

Arguments

g

gadget, connected and running at some speed

hs

descriptor to use for high speed operation

fs

descriptor to use for full or low speed operation

Name

struct usb_configuration — represents one gadget configuration

Synopsis

struct usb_configuration {
const char * label;
struct usb_gadget_strings ** strings;
const struct usb_descriptor_header ** descriptors;
void (* unbind) (struct usb_configuration *);
int (* setup) (struct usb_configuration *,const struct usb_ctrlrequest *);
u8 bConfigurationValue;
u8 iConfiguration;
u8 bmAttributes;
u8 bMaxPower;
struct usb_composite_dev * cdev;
};

Members

label
For diagnostics, describes the configuration.

strings
Tables of strings, keyed by identifiers assigned during 

bind

() and by language IDs provided in control requests.

descriptors
Table of descriptors preceding all function descriptors. Examples include OTG and vendor-specific descriptors.

unbind
Reverses 

bind

; called as a side effect of unregistering the driver which added this configuration.

setup
Used to delegate control requests that aren't handled by standard device infrastructure or directed at a specific interface.

bConfigurationValue
Copied into configuration descriptor.

iConfiguration
Copied into configuration descriptor.

bmAttributes
Copied into configuration descriptor.

bMaxPower
Copied into configuration descriptor.

cdev
assigned by 

usb_add_config

() before calling 

bind

(); this is the device associated with this configuration.

Description

Configurations are building blocks for gadget drivers structured around function drivers. Simple USB gadgets require only one function and one configuration, and handle dual-speed hardware by always providing the same functionality. Slightly more complex
gadgets may have more than one single-function configuration at a given speed; or have configurations that only work at one speed.

Composite devices are, by definition, ones with configurations which include more than one function.

The lifecycle of a usb_configuration includes allocation, initialization of the fields described above, and calling 

usb_add_config

() to set up internal data and bind it to a specific device.
The configuration's 

bind

() method is then used to initialize all the functions and then call 

usb_add_function

() for
them.

Those functions would normally be independent of each other, but that's not mandatory. CDC WMC devices are an example where functions often depend on other functions, with some functions subsidiary to others. Such interdependency may be managed in any way,
so long as all of the descriptors complete by the time the composite driver returns from its 

bind

 routine.

Name

struct usb_composite_driver — groups configurations into a gadget

Synopsis

struct usb_composite_driver {
const char * name;
const char * iProduct;
const char * iManufacturer;
const struct usb_device_descriptor * dev;
struct usb_gadget_strings ** strings;
unsigned needs_serial:1;
int (* unbind) (struct usb_composite_dev *);
void (* disconnect) (struct usb_composite_dev *);
void (* suspend) (struct usb_composite_dev *);
void (* resume) (struct usb_composite_dev *);
};

Members

name
For diagnostics, identifies the driver.

iProduct
Used as iProduct override if 

dev

->iProduct is not set. If NULL value of 

name

 is taken.

iManufacturer
Used as iManufacturer override if 

dev

->iManufacturer is not set. If NULL a default “<system> <release> with <udc>” value will be used.

dev
Template descriptor for the device, including default device identifiers.

strings
tables of strings, keyed by identifiers assigned during 

bind

 and language IDs provided in control requests

needs_serial
set to 1 if the gadget needs userspace to provide a serial number. If one is not provided, warning will be printed.

unbind
Reverses bind; called as a side effect of unregistering this driver.

disconnect
optional driver disconnect method

suspend
Notifies when the host stops sending USB traffic, after function notifications

resume
Notifies configuration when the host restarts USB traffic, before function notifications

Description

Devices default to reporting self powered operation. Devices which rely on bus powered operation should report this in their 

bind

() method.

Before returning from bind, various fields in the template descriptor may be overridden. These include the idVendor/idProduct/bcdDevice values normally to bind the appropriate host side driver, and the three strings (iManufacturer, iProduct, iSerialNumber)
normally used to provide user meaningful device identifiers. (The strings will not be defined unless they are defined in 

dev

 and 

strings

.)
The correct ep0 maxpacket size is also reported, as defined by the underlying controller driver.

Name

struct usb_composite_dev — represents one composite usb gadget

Synopsis

struct usb_composite_dev {
struct usb_gadget * gadget;
struct usb_request * req;
unsigned bufsiz;
struct usb_configuration * config;
};

Members

gadget
read-only, abstracts the gadget's usb peripheral controller

req
used for control responses; buffer is pre-allocated

bufsiz
size of buffer pre-allocated in 

req

config
the currently active configuration

Description

One of these devices is allocated and initialized before the associated device driver's 

bind

 is called.

OPEN ISSUE

it appears that some WUSB devices will need to be built by combining a normal (wired) gadget with a wireless one. This revision of the gadget framework should probably try to make sure doing that won't hurt too much.

One notion for how to handle Wireless USB devices involves

(a) a second gadget here, discovery mechanism TBD, but likely needing separate “register/unregister WUSB gadget” calls; (b) updates to usb_gadget to include flags “is
it wireless”, “is it wired”, plus (presumably in a wrapper structure) bandgroup and PHY info; (c) presumably a wireless_ep wrapping a usb_ep, and reporting wireless-specific parameters like
maxburst and maxsequence; (d) configurations that are specific to wireless links; (e) function drivers that understand wireless configs and will support wireless for (additional) function instances; (f) a function to support association setup (like CBAF),
not necessarily requiring a wireless adapter; (g) composite device setup that can create one or more wireless configs, including appropriate association setup support; (h) more, TBD.

Name

usb_add_function — add a function to a configuration

Synopsis

int usb_add_function (	config,
	function ) ;

struct usb_configuration * config

;

struct usb_function * function

;
 

Arguments

config

the configuration

function

the function being added

Context

single threaded during gadget setup

Description

After initialization, each configuration must have one or more functions added to it. Adding a function involves calling its 

bind

() method to allocate resources such as interface and string
identifiers and endpoints.

This function returns the value of the function's 

bind

, which is zero for success else a negative errno value.

Name

usb_function_deactivate — prevent function and gadget enumeration

Synopsis

int usb_function_deactivate (

function ) ;

struct usb_function * function

;
 

Arguments

function

the function that isn't yet ready to respond

Description

Blocks response of the gadget driver to host enumeration by preventing the data line pullup from being activated. This is normally called during 

bind

() processing to change from the initial “ready
to respond” state, or when a required resource becomes available.

For example, drivers that serve as a passthrough to a userspace daemon can block enumeration unless that daemon (such as an OBEX, MTP, or print server) is ready to handle host requests.

Not all systems support software control of their USB peripheral data pullups.

Returns zero on success, else negative errno.

Name

usb_function_activate — allow function and gadget enumeration

Synopsis

int usb_function_activate (

function ) ;

struct usb_function * function

;
 

Arguments

function

function on which 

usb_function_activate

 was called

Description

Reverses effect of 

usb_function_deactivate

. If no more functions are delaying their activation, the gadget driver will respond to host enumeration procedures.

Returns zero on success, else negative errno.

Name

usb_interface_id — allocate an unused interface ID

Synopsis

int usb_interface_id (	config,
	function ) ;

struct usb_configuration * config

;

struct usb_function * function

;
 

Arguments

config

configuration associated with the interface

function

function handling the interface

Context

single threaded during gadget setup

Description

usb_interface_id

 is called from usb_function.

bind

 callbacks to allocate new interface IDs. The function driver will then store that ID in interface, association, CDC union, and other descriptors.
It will also handle any control requests targeted at that interface, particularly changing its altsetting via

set_alt

. There may also be class-specific or vendor-specific requests to handle.

All interface identifier should be allocated using this routine, to ensure that for example different functions don't wrongly assign different meanings to the same identifier. Note that since interface identifiers are configuration-specific, functions used
in more than one configuration (or more than once in a given configuration) need multiple versions of the relevant descriptors.

Returns the interface ID which was allocated; or -ENODEV if no more interface IDs can be allocated.

Name

usb_add_config — add a configuration to a device.

Synopsis

int usb_add_config (	cdev,
	config,
	bind ) ;

struct usb_composite_dev * cdev

;

struct usb_configuration * config

;

int (*bind) 
(
struct usb_configuration *[code])

;
 

Arguments

cdev

wraps the USB gadget

config

the configuration, with bConfigurationValue assigned

bind

the configuration's bind function

Context

single threaded during gadget setup

Description

One of the main tasks of a composite 

bind

() routine is to add each of the configurations it supports, using this routine.

This function returns the value of the configuration's 

bind

(), which is zero for success else a negative errno value. Binding configurations assigns global resources including string IDs,
and per-configuration resources such as interface IDs and endpoints.

Name

usb_string_id — allocate an unused string ID

Synopsis

int usb_string_id (

cdev ) ;

struct usb_composite_dev * cdev

;
 

Arguments

cdev

the device whose string descriptor IDs are being allocated

Context

single threaded during gadget setup

Description

usb_string_id

() is called from 

bind

 callbacks to allocate string IDs. Drivers for functions, configurations, or gadgets will then store that ID in the appropriate
descriptors and string table.

All string identifier should be allocated using this, 

usb_string_ids_tab

() or 

usb_string_ids_n

() routine, to ensure
that for example different functions don't wrongly assign different meanings to the same identifier.

Name

usb_string_ids_tab — allocate unused string IDs in batch

Synopsis

int usb_string_ids_tab (	cdev,
	str ) ;

struct usb_composite_dev * cdev

;

struct usb_string * str

;
 

Arguments

cdev

the device whose string descriptor IDs are being allocated

str

an array of usb_string objects to assign numbers to

Context

single threaded during gadget setup

Description

usb_string_ids

() is called from 

bind

 callbacks to allocate string IDs. Drivers for functions, configurations, or gadgets will then copy IDs from the string
table to the appropriate descriptors and string table for other languages.

All string identifier should be allocated using this, 

usb_string_id

() or 

usb_string_ids_n

() routine, to ensure that
for example different functions don't wrongly assign different meanings to the same identifier.

Name

usb_string_ids_n — allocate unused string IDs in batch

Synopsis

int usb_string_ids_n (	c,
	n ) ;

struct usb_composite_dev * c

;

unsigned n

;
 

Arguments

c

the device whose string descriptor IDs are being allocated

n

number of string IDs to allocate

Context

single threaded during gadget setup

Description

Returns the first requested ID. This ID and next 

n

-1 IDs are now valid IDs. At least provided that 

n

 is non-zero because
if it is, returns last requested ID which is now very useful information.

usb_string_ids_n

() is called from 

bind

 callbacks to allocate string IDs. Drivers for functions, configurations, or gadgets will then store that ID in the
appropriate descriptors and string table.

All string identifier should be allocated using this, 

usb_string_id

() or 

usb_string_ids_n

() routine, to ensure that
for example different functions don't wrongly assign different meanings to the same identifier.

Name

usb_composite_probe — register a composite driver

Synopsis

int usb_composite_probe (	driver,
	bind ) ;

struct usb_composite_driver * driver

;

int (*bind) 
(
struct usb_composite_dev *cdev[code])

;
 

Arguments

driver

the driver to register

bind

the callback used to allocate resources that are shared across the whole device, such as string IDs, and add its configurations using 

usb_add_config

(). This may fail by returning a negative
errno value; it should return zero on successful initialization.

Context

single threaded during gadget setup

Description

This function is used to register drivers using the composite driver framework. The return value is zero, or a negative errno value. Those values normally come from the driver's 

bind

 method,
which does all the work of setting up the driver to match the hardware.

On successful return, the gadget is ready to respond to requests from the host, unless one of its components invokes 

usb_gadget_disconnect

 while it was binding. That would usually be done in order to wait for some userspace
participation.

Name

usb_composite_unregister — unregister a composite driver

Synopsis

void usb_composite_unregister (

driver ) ;

struct usb_composite_driver * driver

;
 

Arguments

driver

the driver to unregister

Description

This function is used to unregister drivers using the composite driver framework.

Name

usb_composite_setup_continue — Continue with the control transfer

Synopsis

void usb_composite_setup_continue (

cdev ) ;

struct usb_composite_dev * cdev

;
 

Arguments

cdev

the composite device who's control transfer was kept waiting

Description

This function must be called by the USB function driver to continue with the control transfer's data/status stage in case it had requested to delay the data/status stages. A USB function's setup handler (e.g. 

set_alt

) can request
the composite framework to delay the setup request's data/status stages by returning USB_GADGET_DELAYED_STATUS.

Composite Device Functions

At this writing, a few of the current gadget drivers have been converted to this framework. Near-term plans include converting all of them, except for "gadgetfs".

Name

acm_cdc_notify — issue CDC notification to host

Synopsis

int acm_cdc_notify (	acm,
	type,
	value,
	data,
	length ) ;

struct f_acm * acm

;

u8 type

;

u16 value

;

void * data

;

unsigned length

;
 

Arguments

acm

wraps host to be notified

type

notification type

value

Refer to cdc specs, wValue field.

data

data to be sent

length

size of data

Context

irqs blocked, acm->lock held, acm_notify_req non-null

Description

Returns zero on success or a negative errno.

See section 6.3.5 of the CDC 1.1 specification for information

about the only notification we issue

SerialState change.

Name

acm_bind_config — add a CDC ACM function to a configuration

Synopsis

int acm_bind_config (	c,
	port_num ) ;

struct usb_configuration * c

;

u8 port_num

;
 

Arguments

c

the configuration to support the CDC ACM instance

port_num

/dev/ttyGS* port this interface will use

Context

single threaded during gadget setup

Description

Returns zero on success, else negative errno.

Caller must have called 

gserial_setup

() with enough ports to handle all the ones it binds. Caller is also responsible for calling 

gserial_cleanup

()
before module unload.

Name

ecm_bind_config — add CDC Ethernet network link to a configuration

Synopsis

int ecm_bind_config (	c,
	ethaddr[ETH_ALEN] ) ;

struct usb_configuration * c

;

u8 ethaddr[ETH_ALEN]

;
 

Arguments

c

the configuration to support the network link

ethaddr[ETH_ALEN]

a buffer in which the ethernet address of the host side side of the link was recorded

Context

single threaded during gadget setup

Description

Returns zero on success, else negative errno.

Caller must have called 

gether_setup

(). Caller is also responsible for calling 

gether_cleanup

() before module unload.

Name

geth_bind_config — add CDC Subset network link to a configuration

Synopsis

int geth_bind_config (	c,
	ethaddr[ETH_ALEN] ) ;

struct usb_configuration * c

;

u8 ethaddr[ETH_ALEN]

;
 

Arguments

c

the configuration to support the network link

ethaddr[ETH_ALEN]

a buffer in which the ethernet address of the host side side of the link was recorded

Context

single threaded during gadget setup

Description

Returns zero on success, else negative errno.

Caller must have called 

gether_setup

(). Caller is also responsible for calling 

gether_cleanup

() before module unload.

Name

obex_bind_config — add a CDC OBEX function to a configuration

Synopsis

int obex_bind_config (	c,
	port_num ) ;

struct usb_configuration * c

;

u8 port_num

;
 

Arguments

c

the configuration to support the CDC OBEX instance

port_num

/dev/ttyGS* port this interface will use

Context

single threaded during gadget setup

Description

Returns zero on success, else negative errno.

Caller must have called 

gserial_setup

() with enough ports to handle all the ones it binds. Caller is also responsible for calling 

gserial_cleanup

()
before module unload.

Name

gser_bind_config — add a generic serial function to a configuration

Synopsis

int gser_bind_config (	c,
	port_num ) ;

struct usb_configuration * c

;

u8 port_num

;
 

Arguments

c

the configuration to support the serial instance

port_num

/dev/ttyGS* port this interface will use

Context

single threaded during gadget setup

Description

Returns zero on success, else negative errno.

Caller must have called 

gserial_setup

() with enough ports to handle all the ones it binds. Caller is also responsible for calling 

gserial_cleanup

()
before module unload.

Chapter 4. Peripheral Controller Drivers

The first hardware supporting this API was the NetChip 2280 controller, which supports USB 2.0 high speed and is based on PCI. This is the 

net2280

 driver module. The driver supports Linux kernel versions 2.4 and 2.6; contact
NetChip Technologies for development boards and product information.

Other hardware working in the "gadget" framework includes: Intel's PXA 25x and IXP42x series processors (

pxa2xx_udc

), Toshiba TC86c001 "Goku-S" (

goku_udc

), Renesas SH7705/7727 (

sh_udc

),
MediaQ 11xx (

mq11xx_udc

), Hynix HMS30C7202 (

h7202_udc

), National 9303/4 (

n9604_udc

), Texas Instruments OMAP (

omap_udc

), Sharp LH7A40x (

lh7a40x_udc

),
and more. Most of those are full speed controllers.

At this writing, there are people at work on drivers in this framework for several other USB device controllers, with plans to make many of them be widely available.

A partial USB simulator, the 

dummy_hcd

 driver, is available. It can act like a net2280, a pxa25x, or an sa11x0 in terms of available endpoints and device speeds; and it simulates control, bulk, and to some extent interrupt transfers.
That lets you develop some parts of a gadget driver on a normal PC, without any special hardware, and perhaps with the assistance of tools such as GDB running with User Mode Linux. At least one person has expressed interest in adapting that approach, hooking
it up to a simulator for a microcontroller. Such simulators can help debug subsystems where the runtime hardware is unfriendly to software development, or is not yet available.

Support for other controllers is expected to be developed and contributed over time, as this driver framework evolves.

Chapter 5. Gadget Drivers

In addition to Gadget Zero (used primarily for testing and development with drivers for usb controller hardware), other gadget drivers exist.

There's an ethernet gadget driver, which implements one of the most useful Communications Device Class (CDC) models. One of the standards for cable modem interoperability even
specifies the use of this ethernet model as one of two mandatory options. Gadgets using this code look to a USB host as if they're an Ethernet adapter. It provides access to a network where the gadget's CPU is one host, which could easily be bridging, routing,
or firewalling access to other networks. Since some hardware can't fully implement the CDC Ethernet requirements, this driver also implements a "good parts only" subset of CDC Ethernet. (That subset doesn't advertise itself as CDC Ethernet, to avoid creating
problems.)

Support for Microsoft's RNDIS protocol has been contributed by Pengutronix and Auerswald GmbH. This is like CDC Ethernet, but it runs on more slightly USB hardware (but less than the CDC subset). However, its main claim
to fame is being able to connect directly to recent versions of Windows, using drivers that Microsoft bundles and supports, making it much simpler to network with Windows.

There is also support for user mode gadget drivers, using gadgetfs. This provides a User Mode API that presents each endpoint as a single file descriptor. I/O is done using normal read() and read() calls.
Familiar tools like GDB and pthreads can be used to develop and debug user mode drivers, so that once a robust controller driver is available many applications for it won't require new kernel mode software. Linux 2.6 Async I/O (AIO) support
is available, so that user mode software can stream data with only slightly more overhead than a kernel driver.

There's a USB Mass Storage class driver, which provides a different solution for interoperability with systems such as MS-Windows and MacOS. That File-backed Storage driver uses a file or block device as backing store
for a drive, like the 

loop

 driver. The USB host uses the BBB, CB, or CBI versions of the mass storage class specification, using transparent SCSI commands to access the data from the backing store.

There's a "serial line" driver, useful for TTY style operation over USB. The latest version of that driver supports CDC ACM style operation, like a USB modem, and so on most hardware it can interoperate easily with MS-Windows. One interesting use of that
driver is in boot firmware (like a BIOS), which can sometimes use that model with very small systems without real serial lines.

Support for other kinds of gadget is expected to be developed and contributed over time, as this driver framework evolves.

Chapter 6. USB On-The-GO (OTG)

USB OTG support on Linux 2.6 was initially developed by Texas Instruments for OMAP 16xx and 17xx series processors. Other OTG systems should work in similar ways, but the hardware level details
could be very different.

Systems need specialized hardware support to implement OTG, notably including a special Mini-AB jack and associated transciever to support Dual-Roleoperation: they can act either
as a host, using the standard Linux-USB host side driver stack, or as a peripheral, using this "gadget" framework. To do that, the system software relies on small additions to those programming interfaces, and on a new internal component (here called an "OTG
Controller") affecting which driver stack connects to the OTG port. In each role, the system can re-use the existing pool of hardware-neutral drivers, layered on top of the controller driver interfaces (usb_bus or usb_gadget).
Such drivers need at most minor changes, and most of the calls added to support OTG can also benefit non-OTG products.

Gadget drivers test the is_otg flag, and use it to determine whether or not to include an OTG descriptor in each of their configurations.

Gadget drivers may need changes to support the two new OTG protocols, exposed in new gadget attributes such as b_hnp_enable flag. HNP support should be reported through a user interface (two LEDs could suffice), and
is triggered in some cases when the host suspends the peripheral. SRP support can be user-initiated just like remote wakeup, probably by pressing the same button.

On the host side, USB device drivers need to be taught to trigger HNP at appropriate moments, using 

usb_suspend_device()

. That also conserves battery power, which is useful even for non-OTG configurations.

Also on the host side, a driver must support the OTG "Targeted Peripheral List". That's just a whitelist, used to reject peripherals not supported with a given Linux OTG host. This whitelist is product-specific; each product must
modify 

otg_whitelist.h

 to match its interoperability specification.

Non-OTG Linux hosts, like PCs and workstations, normally have some solution for adding drivers, so that peripherals that aren't recognized can eventually be supported. That approach is unreasonable for consumer products that may never have their firmware
upgraded, and where it's usually unrealistic to expect traditional PC/workstation/server kinds of support model to work. For example, it's often impractical to change device firmware once the product has been distributed, so driver bugs can't normally be fixed
if they're found after shipment.

Additional changes are needed below those hardware-neutral usb_bus and usb_gadget driver interfaces; those aren't discussed here in any detail. Those affect the hardware-specific
code for each USB Host or Peripheral controller, and how the HCD initializes (since OTG can be active only on a single port). They also involve what may be called an OTG Controller Driver, managing the OTG transceiver
and the OTG state machine logic as well as much of the root hub behavior for the OTG port. The OTG controller driver needs to activate and deactivate USB controllers depending on the relevant device role. Some related changes were needed inside usbcore, so
that it can identify OTG-capable devices and respond appropriately to HNP or SRP protocols.