Real Time Clock (RTC) Drivers for Linux

/*将这篇英文原文的RTC放于此，方便阅读和后续的翻译，仅学习使用！*/

Real Time Clock (RTC) Drivers for Linux

=======================================

When Linux developers talk about a "Real Time Clock" they usually mean something that tracks wall clock time and is battery backed so that it works even with system power off. Such clocks will normally
not track the local time zone or daylight savings time -- unless they dual boot with MS-Windows -- but will instead be set to Coordinated Universal Time (UTC formerly "Greenwich Mean Time").

The newest non-PC hardware tends to just count seconds like the time (2) system call reports but RTCs also very commonly represent time using the Gregorian calendar and 24 hour time as reported
by gmtime (3).

Linux has two largely-compatible userspace RTC API families you may need to know about:

* /dev/rtc ... is the RTC provided by PC compatible systems so it's not very portable to non-x86 systems.

* /dev/rtc0 /dev/rtc1 ... are part of a framework that's supported by a wide variety of RTC chips on all systems.

Programmers need to understand that the PC/AT functionality is not always available and some systems can do much more. That is the RTCs use the same API to make requests in both RTC frameworks
(using different filenames of course) but the hardware may not offer the same functionality. For example not every RTC is hooked up to an IRQ so they can 't all issue alarms; and where standard PC RTCs can only issue an alarm up to 24 hours in the future
other hardware may be able to schedule one any time in the upcoming century.

Old PC/AT-Compatible driver: /dev/rtc

--------------------------------------

All PCs (even Alpha machines) have a Real Time Clock built into them . Usually they are built into the chipset of the computer but some may actually have a Motorola MC146818 (or clone) on the
board. This is the clock that keeps the date and time while your computer is turned off.

ACPI has standardized that MC146818 functionality and extended it in a few ways (enabling longer alarm periods and wake-from-hibernate). That
functionality is NOT exposed in the old driver.

However it can also be used to generate signals from a slow 2Hz to a relatively fast 8192Hz in increments of powers of two. These
signals are reported by interrupt number 8. (Oh ! So *that* is what IRQ 8 is for...) It can also function as a 24hr alarm
raising IRQ 8 when the alarm goes off. The alarm can also be programmed to only check any subset of the three programmable
values meaning that it could be set to ring on the 30th second of the 30th minute of every hour for example. The clock can
also be set to generate an interrupt upon every clock update thus generating a 1Hz signal.

The interrupts are reported via /dev/rtc (major 10 minor 135 read only character device) in the form of an unsigned long. The
low byte contains the type of interrupt (update-done alarm-rang or periodic) that was raised and the remaining bytes contain the number of interrupts since the last read. Status information is reported through the pseudo-file /proc/driver/rtc if the /proc
filesystem was enabled. The driver has

built in locking so that only one process is allowed to have the /dev/rtc interface open at a time.

A user process can monitor these interrupts by doing a read (2) or a select (2) on /dev/rtc -- either will block/stop the user
process until the next interrupt is received. This is useful for things like reasonably high frequency data acquisition where one doesn 't want to burn up 100% CPU by polling gettimeofday etc. etc.

At high frequencies or under high loads the user process should check the number of interrupts received since the last read to
determine if there has been any interrupt "pileup" so to speak. Just for reference a typical 486-33 running a tight read loop on /dev/rtc will start to suffer occasional interrupt pileup (i.e. > 1 IRQ event since last read) for frequencies above 1024Hz.
So you really should check the high bytes of the value you read especially at frequencies above that of the normal timer interrupt which is 100Hz.

Programming and/or enabling interrupt frequencies greater than 64Hz is only allowed by root. This is perhaps a bit conservative
but we don 't want

an evil user generating lots of IRQs on a slow 386sx-16 where it might have a negative impact on performance. This 64Hz limit can be changed by writing a different value to /proc/sys/dev/rtc/max-user-freq. Note that the interrupt handler is only a few lines
of code to minimize any possibility of this effect.

Also if the kernel time is synchronized with an external source the kernel will write the time back to the CMOS clock every
11 minutes. In the process of doing this the kernel briefly turns off RTC periodic interrupts so be aware of this if you are doing serious work. If you don 't synchronize the kernel time with an external source (via ntp or whatever) then the kernel will
keep its hands off the RTC allowing you exclusive access to the device for your applications.

The alarm and/or interrupt frequency are programmed into the RTC via various ioctl (2) calls as listed in ./include/linux/rtc.h

Rather than write 50 pages describing the ioctl () and so on it is perhaps more useful to include a small test program that demonstrates how to use them and demonstrates the features of the driver.
This is probably a lot more useful to people interested in writing applications that will be using this driver. See the code at the end of this document.

(The original /dev/rtc driver was written by Paul Gortmaker.)

New portable "RTC Class" drivers: /dev/rtcN

--------------------------------------------

Because Linux supports many non-ACPI and non-PC platforms some of which have more than one RTC style clock it needed a more
portable solution

than expecting a single battery-backed MC146818 clone on every system . Accordingly a new "RTC Class" framework has been defined. It offers three different userspace interfaces:

* /dev/rtcN ... much the same as the older /dev/rtc interface

* /sys/class/rtc/rtcN ... sysfs attributes support readonly access to some RTC attributes.

* /proc/driver/rtc ... the first RTC (rtc0) may expose itself using a procfs interface. More information is (currently)
shown here than through sysfs.

The RTC Class framework supports a wide variety of RTCs ranging from those integrated into embeddable system-on-chip (SOC) processors
to discrete chips using I2C SPI or some other bus to communicate with the host CPU . There's even support for PC-style RTCs ... including the features exposed on newer PCs through ACPI.

The new framework also removes the "one RTC per system" restriction. For example maybe the low-power battery-backed RTC is a
discrete I2C chip but a high functionality RTC is integrated into the SOC. That system might read the system clock from the discrete RTC but use the integrated one for all other tasks because of its greater functionality.

SYSFS INTERFACE

---------------

The sysfs interface under /sys/class/rtc/rtcN provides access to various rtc attributes without requiring the use of ioctls. All dates and times are in the RTC's timezone rather than in system time.

date: RTC-provided date

hctosys: 1 if the RTC provided the system time at boot via the CONFIG_RTC_HCTOSYS kernel option 0 otherwise

max_user_freq: The maximum interrupt rate an unprivileged user may request from this RTC.

name: The name of the RTC corresponding to this sysfs directory

sice_epoch : The number of seconds since the epoch according to the RTC

time: RTC-provided time

wakealarm : The time at which the clock will generate a system wakeup event. This is a one shot wakeup event so must be reset after wake if a daily wakeup is required. Format is either seconds since the epoch or if there's a leading + seconds in the future.

IOCTL INTERFACE

---------------

The ioctl () calls supported by /dev/rtc are also supported by the RTC class framework. However because the chips and systems are not standardized

some PC/AT functionality might not be provided. And in the same way some newer features -- including those enabled by ACPI -- are exposed by the

RTC class framework but can 't be supported by the older driver.

* RTC_RD_TIME RTC_SET_TIME ... every RTC supports at least reading time returning the result as a Gregorian calendar
date and 24 hour

wall clock time. To be most useful this time may also be updated.

* RTC_AIE_ON RTC_AIE_OFF RTC_ALM_SET RTC_ALM_READ ... when the RTC is connected to an IRQ line it can often issue an alarm IRQ up to 24 hours in the future. (Use RTC_WKALM_* by preference.)

* RTC_WKALM_SET RTC_WKALM_RD ... RTCs that can issue alarms beyond the next 24 hours use a slightly more powerful API which supports setting the longer alarm time and enabling its IRQ using a single request (using the same model as EFI firmware).

* RTC_UIE_ON RTC_UIE_OFF ... if the RTC offers IRQs the RTC framework will emulate this mechanism .

* RTC_PIE_ON RTC_PIE_OFF RTC_IRQP_SET RTC_IRQP_READ ... these icotls are emulated via a kernel hrtimer.

In many cases the RTC alarm can be a system wake event used to force Linux out of a low power sleep state (or hibernation)
back to a fully operational state. For example a system could enter a deep power saving state until it's time to execute some scheduled tasks.

Note that many of these ioctls are handled by the common rtc-dev interface. Some common examples:

* RTC_RD_TIME RTC_SET_TIME : the read_time/set_time functions will be called with appropriate values.

* RTC_ALM_SET RTC_ALM_READ RTC_WKALM_SET RTC_WKALM_RD: gets or sets the alarm rtc_timer. May call the set_alarm driver function.

* RTC_IRQP_SET RTC_IRQP_READ: These are emulated by the generic code.

* RTC_PIE_ON RTC_PIE_OFF: These are also emulated by the generic code.

If all else fails check out the rtc-test.c driver!

-------------------- 8< ---------------- 8< ----------------------------- 
/* 
 *      Real Time Clock Driver Test/Example Program 
 * 
 *      Compile with : 
 *       gcc -s -Wall -Wstrict-prototypes rtctest.c -o rtctest 
 * 
 *      Copyright  (C) 1996  Paul Gortmaker. 
 * 
 *      Released under the GNU General Public License  version 2 
 *      included herein by reference. 
 * 
 */ 
#include <stdio.h> 
#include <linux/rtc.h> 
#include <sys/ioctl.h> 
#include <sys/time.h> 
#include <sys/types.h> 
#include <fcntl.h> 
#include <unistd.h> 
#include <stdlib .h> 
#include <errno.h> 
/* 
 * This expects the new RTC class driver framework  working with 
 * clocks that will often not be clones of what the PC-AT had. 
 * Use the command line to specify another RTC if you need one. 
 */ 
static const char default_rtc [] = "/dev/rtc0"; 

int main (int argc  char **argv) 
{ 
 int i  fd  retval  irqcount = 0; 
unsigned long tmp  data; 
 struct rtc_time rtc_tm; 
 const char *rtc = default_rtc; 

 switch  (argc)  { 
 case 2: 
  rtc = argv [1]; 
  /* FALLTHROUGH */ 
 case 1: 
  break; 
 default: 
  fprintf (stderr  "usage:  rtctest  [rtcdev]\n"); 
  return 1; 
  } 

 fd = open (rtc  O_RDONLY); 

 if  (fd ==  -1)  { 
  perror (rtc); 
  exit (errno); 
  } 

 fprintf (stderr  "\n\t\t\tRTC Driver Test Example.\n\n"); 
 /* Turn on update interrupts  (one per second) */ 
 retval = ioctl (fd  RTC_UIE_ON  0); 
 if  (retval == -1)  { 
  if  (errno == ENOTTY)  { 
   fprintf (stderr 
    "\n...Update IRQs not supported.\n"); 
   goto test_READ; 
   } 
  perror ("RTC_UIE_ON ioctl"); 
  exit (errno); 
  } 

 fprintf (stderr  "Counting 5 update  (1/sec) interrupts from reading %s:" 
   rtc); 
 fflush (stderr); 
 for  (i=1; i<6; i++)  { 
  /* This read will block */ 
  retval = read (fd  &data  sizeof (unsigned long)); 
  if  (retval == -1)  { 
   perror ("read"); 
   exit (errno); 
   } 
  fprintf (stderr  " %d" i); 
  fflush (stderr); 
  irqcount++; 
 } 

 fprintf (stderr  "\nAgain  from using select (2) on /dev/rtc:"); 
 fflush (stderr); 
 for  (i=1; i<6; i++)  { 
  struct timeval tv =  {5  0};     /* 5 second timeout on select */ 
  fd_set readfds; 

  FD_ZERO (&readfds); 
  FD_SET (fd  &readfds); 
  /* The select will wait until an RTC interrupt happens. */ 
  retval = select (fd+1  &readfds  NULL  NULL  &tv); 
  if  (retval == -1)  { 
          perror ("select"); 
          exit (errno); 
   } 
  /* This read won 't block unlike the select-less case above. */ 
  retval = read (fd  &data  sizeof (unsigned long)); 
  if  (retval == -1)  { 
          perror ("read"); 
          exit (errno); 
   } 
  fprintf (stderr  " %d" i); 
  fflush (stderr); 
  irqcount++; 
  } 

 /* Turn off update interrupts */ 
 retval = ioctl (fd  RTC_UIE_OFF  0); 
 if  (retval == -1)  { 
  perror ("RTC_UIE_OFF ioctl"); 
  exit (errno); 
  } 

test_READ: 
 /* Read the RTC time/date */ 
 retval = ioctl (fd  RTC_RD_TIME  &rtc_tm); 
 if  (retval == -1)  { 
  perror ("RTC_RD_TIME ioctl"); 
  exit (errno); 
  } 

 fprintf (stderr  "\n\nCurrent RTC date/time is %d-%d-%d  %02d:%02d:%02d.\n" 
  rtc_tm .tm_mday  rtc_tm .tm_mon + 1  rtc_tm .tm_year + 1900 
  rtc_tm .tm_hour  rtc_tm .tm_min  rtc_tm .tm_sec); 

 /* Set the alarm to 5 sec in the future  and check for rollover */ 
 rtc_tm .tm_sec += 5; 
 if  (rtc_tm .tm_sec >= 60)  { 
rtc_tm .tm_sec %= 60; 
  rtc_tm .tm_min++; 
  } 
 if  (rtc_tm .tm_min == 60)  { 
  rtc_tm .tm_min = 0; 
  rtc_tm .tm_hour++; 
  } 
 if  (rtc_tm .tm_hour == 24) 
  rtc_tm .tm_hour = 0; 

 retval = ioctl (fd  RTC_ALM_SET  &rtc_tm); 
 if  (retval == -1)  { 
  if  (errno == ENOTTY)  { 
   fprintf (stderr 
    "\n...Alarm IRQs not supported.\n"); 
   goto test_PIE; 
   } 
  perror ("RTC_ALM_SET ioctl"); 
  exit (errno); 
  } 

 /* Read the current alarm settings */ 
 retval = ioctl (fd  RTC_ALM_READ  &rtc_tm); 
 if  (retval == -1)  { 
  perror ("RTC_ALM_READ ioctl"); 
  exit (errno); 
  } 

 fprintf (stderr  "Alarm time now set to %02d:%02d:%02d.\n" 
  rtc_tm .tm_hour  rtc_tm .tm_min  rtc_tm .tm_sec); 

 /* Enable alarm interrupts */ 
 retval = ioctl (fd  RTC_AIE_ON  0); 
 if  (retval == -1)  { 
  perror ("RTC_AIE_ON ioctl"); 
  exit (errno); 
  } 

 fprintf (stderr  "Waiting 5 seconds for alarm ..."); 
 fflush (stderr); 
 /* This blocks until the alarm ring causes an interrupt */ 
 retval = read (fd  &data  sizeof (unsigned long)); 
 if  (retval == -1)  { 
  perror ("read"); 
  exit (errno); 
  } 
 irqcount++; 
 fprintf (stderr  " okay. Alarm rang.\n"); 
/* Disable alarm interrupts */ 
 retval = ioctl (fd  RTC_AIE_OFF  0); 
 if  (retval == -1)  { 
  perror ("RTC_AIE_OFF ioctl"); 
  exit (errno); 
  } 

test_PIE : 
 /* Read periodic IRQ rate */ 
 retval = ioctl (fd  RTC_IRQP_READ  &tmp); 
 if  (retval == -1)  { 
  /* not all RTCs support periodic IRQs */ 
  if  (errno == ENOTTY)  { 
   fprintf (stderr  "\nNo periodic IRQ support\n"); 
   goto done; 
   } 
  perror ("RTC_IRQP_READ ioctl"); 
  exit (errno); 
  } 
 fprintf (stderr  "\nPeriodic IRQ rate is %ldHz.\n"  tmp); 

 fprintf (stderr  "Counting 20 interrupts at:"); 
 fflush (stderr); 

 /* The frequencies 128Hz  256Hz   ... 8192Hz are only allowed for root. */ 
 for  (tmp=2; tmp<=64; tmp*=2)  { 

  retval = ioctl (fd  RTC_IRQP_SET  tmp); 
  if  (retval == -1)  { 
   /* not all RTCs can change their periodic IRQ rate */ 
   if  (errno == ENOTTY)  { 
    fprintf (stderr 
     "\n...Periodic IRQ rate is fixed\n"); 
    goto done; 
    } 
   perror ("RTC_IRQP_SET ioctl"); 
   exit (errno); 
   } 

  fprintf (stderr  "\n%ldHz:\t"  tmp); 
  fflush (stderr); 

  /* Enable periodic interrupts */ 
  retval = ioctl (fd  RTC_PIE_ON  0); 
  if  (retval == -1)  { 
   perror ("RTC_PIE_ON ioctl"); 
   exit (errno); 
   } 
for  (i=1; i<21; i++)  { 
   /* This blocks */ 
   retval = read (fd  &data  sizeof (unsigned long)); 
   if  (retval == -1)  { 
    perror ("read"); 
    exit (errno); 
    } 
   fprintf (stderr  " %d" i); 
   fflush (stderr); 
   irqcount++; 
   } 

  /* Disable periodic interrupts */ 
  retval = ioctl (fd  RTC_PIE_OFF  0); 
  if  (retval == -1)  { 
   perror ("RTC_PIE_OFF ioctl"); 
   exit (errno); 
   } 
  } 

done: 
 fprintf (stderr  "\n\n\t\t\t *** Test complete ***\n"); 

 close (fd); 
 return 0; 
}