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U_BOOT_CMD 命令数据结构

2012-03-25 01:18 483 查看

U_BOOT_CMD 命令数据结构

2010年08月19日星期四 18:33

The user interface to U-Boot consists of a command line interpreter (CLI), much like a Linux shell prompt. When connected via a serial line you can interactively enter commands and
see the results.
在Uboot的doc目录下的README.commands文件说明如下：

Commands are added to U-Boot by creating a new command structure.

This is done by first including command.h
Then using the U_BOOT_CMD() macro to fill in a cmd_tbl_t struct.
U_BOOT_CMD(name,maxargs,repeatable,command,"usage","help")
name: is the name of the commad. THIS IS NOT a string.

maxargs: the maximumn numbers of arguments this function takes

command: Function pointer (*cmd)(struct cmd_tbl_s *, int, int, char *[ ]);

usage: Short description. This is a string

help: long description. This is a string

**** Behind the scene ******
The structure created is named with a special prefix (__u_boot_cmd_)

and placed by the linker in a special section.
This makes it possible for the final link to extract all commands

compiled into any object code and construct a static array so the

command can be found in an array starting at __u_boot_cmd_start.
If a new board is defined do not forget to define the command section

by writing in u-boot.lds ($(TOPDIR)/board/boardname/u-boot.lds) these

3 lines:
__u_boot_cmd_start = .;

.u_boot_cmd : { *(.u_boot_cmd) }

__u_boot_cmd_end = .;

U-boot的命令用struct cmd_tbl_t来实现。cmd_tbl_t的主要数据成分是命令名称(name)和命令处理函数(cmd)，此外还包括最大参数个数(maxargs),是否可重复执行(repeatable)，使用方法和帮助信息(usage,help)等。这个数据结构在文件include/command.h中定义：

#define Struct_Section __attribute__ ((unused,section (".u_boot_cmd")))

#define U_BOOT_CMD(name,maxargs,rep,cmd,usage,help) \

cmd_tbl_t __u_boot_cmd_##name Struct_Section = {#name, maxargs, rep, cmd, usage, help}
举例如下：

bootm命令定义如下：

U_BOOT_CMD(

bootm, CFG_MAXARGS, 1, do_bootm,

"bootm - boot application image from memory\n",

"[addr [arg ...]]\n - boot application image stored in memory\n"

"\tpassing arguments 'arg ...'; when booting a Linux kernel,\n"

"\t'arg' can be the address of an initrd image\n"

);

用宏定义替换后就是：

cmd_tbl_t __u_boot_cmd_bootm __attribute__ ((unused,section (".u_boot_cmd")))=

{

bootm，

CFG_MAXARGS，

1，

do_bootm，

"bootm - boot application image from memory\n",

"[addr [arg ...]]\n - boot application image stored in memory\n"

"\tpassing arguments 'arg ...'; when booting a Linux kernel,\n"

"\t'arg' can be the address of an initrd image\n"

}

这样就为bootm命令定义了一个cmd_tbl_t 结构。
但__attribute__ ((unused,section (".u_boot_cmd")))又是实现什么功能呢？基于什么考虑呢？

它是用来定义用户的命令, 每当初始化这样的一条命令, 就将在.u_boot_cmd段中增加一段数据。以便于find_cmd函数查找命令。u-boot中的readme文件对这个功能是这样描述的： construct a static array so the command can be found in an array starting at __u_boot_cmd_start.

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