U-Boot | How to add a new command to u-boot console?

U-boot comes with a lot of stock commands which one can run on the u-boot console similar to the Linux console commands like 'ls'. The source for every command can be found under 'common/' directory with file names starting with 'cmd_'. However, not all commands are enabled by default.

A user may either want to enable a command the stock or add his/her own command. Both are possible in just few simple steps.

Note: You can check the list of commands already enabled from the commands list from the u-boot console by running the command 'help' or simply '?'.

1. Enabling a command from the stock
From the code, you can open 'common/Makefile' and under the '# command' section you can find the list of all the commands masked with config flags 'CONFIG_*'. To enable a command, you have to simply #define the corresponding flag under the 'include/configs/<board>.h' file and build the source. Now, you can see the command in the list of commands by running 'help'.

Example:
To enable a command 'source', in the 'common/Makefile', you can find
obj-$(CONFIG_CMD_SOURCE) += cmd_source.o

Simply include the corresponding flag in 'include/configs/<board>.h' file as follows
#define CONFIG_CMD_SOURCE

or in another ways, simply replace $(CONFIG_CMD_SOURCE) with 'y' as follows.
obj-y += cmd_source.o
will enable the command irrespective of any flags defined. However, the first method is most reliable and controllable from the board.h config file.

2. Adding and enabling a new command of your own
In this case, there are three things to be done.
  • Write an API(function) that implements your command.
  • Register your function to the command list.
  • Enable your command.
Write an API that implements your command: First thing is to write an API that implements your expected functionality. But, the API should follow a fixed prototype.

/* Prototype */
int do_funcname(cmd_tbl_t *cmdtp, int flag, int argc, char *const argv[]) 
where,
*cmdtp - Command table pointer (comparable to function vector table)
flag - Unused
argc - Argument count, including command name itself
argv[] -  Array of arguments (string)

Register your API to the command table: Registering your command to the command table is necessary for both, listed and command-line arguments to be properly passed to your API.
U-Boot provides the following prototype to register your API.

/* Prototype */
U_BOOT_CMD(_name, _maxargs, _rep, _cmd, _usage, _help)
where,
_name - Command Name (the name you want to run the command)
_maxargs - Maximum number of arguments
_rep - Autorepeat allowed? (0-No/1-Yes)
_cmd - Implementation function (pointer to your API)
_usage - Usage - Short help message (string)
_help - Long help message (string)

Note: Both the above prototypes are available in 'include/command.h' and hence need to include 'command.h' to your command file header.

Enable your command: Finally, all set to have your own command except one last thing, enabling them. For that, you have to follow the same method as the first case as follow.
obj-y += <filename>.o
or have a new flag defined, say CONFIG_CMD_FILENAME and
obj-$(CONFIG_CMD_FILENAME) += <filename>.o


Reference:
1. doc/README.commands in u-boot source.
2. xillybus.com

Disclaimer:
The above article doesn't give you any guarantee and the sole purpose of this article is to share my learning in the way I understood. Any comments to refine this article are welcome with great pleasure. Please report any breaking link by commenting below.

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