#define ll_entry_declare(_type, _name, _list)				\
	_type _u_boot_list_2_##_list##_2_##_name __aligned(4)		\
			__attribute__((unused,				\
			section(".u_boot_list_2_"#_list"_2_"#_name)))

#define U_BOOT_CMD_MKENT_COMPLETE(_name, _maxargs, _rep, _cmd,		\
				_usage, _help, _comp)			\
		{ #_name, _maxargs, _rep, _cmd, _usage,			\
			_CMD_HELP(_help) _CMD_COMPLETE(_comp) }

#define U_BOOT_CMD_MKENT(_name, _maxargs, _rep, _cmd, _usage, _help)	\
	U_BOOT_CMD_MKENT_COMPLETE(_name, _maxargs, _rep, _cmd,		\
					_usage, _help, NULL)

#define U_BOOT_CMD_COMPLETE(_name, _maxargs, _rep, _cmd, _usage, _help, _comp) \
	ll_entry_declare(cmd_tbl_t, _name, cmd) =			\
		U_BOOT_CMD_MKENT_COMPLETE(_name, _maxargs, _rep, _cmd,	\
						_usage, _help, _comp);

#define U_BOOT_CMD(_name, _maxargs, _rep, _cmd, _usage, _help)		\
	U_BOOT_CMD_COMPLETE(_name, _maxargs, _rep, _cmd, _usage, _help, NULL)

/*
 * Usage
 * gcc -E uboot_cmd.c
 */
int main(int argc, char *argv[])
{
	U_BOOT_CMD(mmcinfo, 1, 0, do_mmcinfo, "short help", "long help");
	return 0;
}

#if 0
typedef struct cmd_tbl_s	cmd_tbl_t;
struct cmd_tbl_s {
	char		*name;		/* Command Name			*/
	int		maxargs;	/* maximum number of arguments	*/
	int		repeatable;	/* autorepeat allowed?		*/
					/* Implementation function	*/
	int		(*cmd)(struct cmd_tbl_s *, int, int, char * const []);
	char		*usage;		/* Usage message	(short)	*/
#ifdef	CONFIG_SYS_LONGHELP
	char		*help;		/* Help  message	(long)	*/
#endif
#ifdef CONFIG_AUTO_COMPLETE
	/* do auto completion on the arguments */
	int		(*complete)(int argc, char * const argv[], char last_char, int maxv, char *cmdv[]);
#endif
};

所以宏U_BOOT_CMD(mmcinfo, 1, 0, do_mmcinfo, "short help", "long help");定义了下面这样一个结构体
 cmd_tbl_t _u_boot_list_2_cmd_2_mmcinfo __aligned(4) __attribute__((unused, section(".u_boot_list_2_""cmd""_2_""mmcinfo"))) = { "mmcinfo", 1, 0, do_mmcinfo, "short help", _CMD_HELP("long help") _CMD_COMPLETE(NULL) };;
#endif