doc: expand README.commands

Describe U_BOOT_CMD_COMPLETE.
Describe the arguments of U_BOOT_CMD and U_BOOT_CMD_COMPLETE.
Describe the arguments of the command function.
Describe the arguments of the completion function.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>

doc/README.commands

@@ -1,19 +1,83 @@
+Command definition
+------------------
 
 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.
+This is done by first including command.h, then using the U_BOOT_CMD() or the
+U_BOOT_CMD_COMPLETE macro to fill in a cmd_tbl_t struct.
 
-U_BOOT_CMD(name,maxargs,repeatable,command,"usage","help")
+U_BOOT_CMD(name, maxargs, repeatable, command, "usage", "help")
+U_BOOT_CMD_COMPLETE(name, maxargs, repeatable, command, "usage, "help", comp)
 
-name:	 is the name of the commad. THIS IS NOT a string.
-maxargs: the maximum number of arguments this function takes
-repeatable: either 0 or 1 to indicate if autorepeat is allowed
-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
+name:		The name of the command. THIS IS NOT a string.
 
+maxargs:	The maximum number of arguments this function takes including
+		the command itself.
 
-**** Behind the scene ******
+repeatable:	Either 0 or 1 to indicate if autorepeat is allowed.
+
+command:	Pointer to the command function. This is the function that is
+		called when the command is issued.
+
+usage:		Short description. This is a string.
+
+help:		Long description. This is a string. The long description is
+		only available if CONFIG_SYS_LONGHELP is defined.
+
+comp:		Pointer to the completion function. May be NULL.
+		This function is called if the user hits the TAB key while
+		entering the command arguments to complete the entry. Command
+		completion is only available if CONFIG_AUTO_COMPLETE is defined.
+
+Command function
+----------------
+
+The commmand function pointer has to be of type
+int (*cmd)(struct cmd_tbl_s *cmdtp, int flag, int argc, const char *argv[]);
+
+cmdtp:		Table entry describing the command (see above).
+
+flag:		A bitmap which may contain the following bit:
+		CMD_FLAG_REPEAT - The last command is repeated.
+		CMD_FLAG_BOOTD  - The command is called by the bootd command.
+		CMD_FLAG_ENV    - The command is called by the run command.
+
+argc:		Number of arguments including the command.
+
+argv:		Arguments.
+
+Allowable return value are:
+
+CMD_SUCCESS	The command was successfully executed.
+
+CMD_FAILURE	The command failed.
+
+CMD_RET_USAGE	The command was called with invalid parameters. This value
+		leads to the display of the usage string.
+
+Completion function
+-------------------
+
+The completion function pointer has to be of type
+int (*complete)(int argc, char *const argv[], char last_char,
+		int maxv, char *cmdv[]);
+
+argc:		Number of arguments including the command.
+
+argv:		Arguments.
+
+last_char:	The last character in the command line buffer.
+
+maxv:		Maximum number of possible completions that may be returned by
+		the function.
+
+cmdv:		Used to return possible values for the last argument. The last
+		possible completion must be followed by NULL.
+
+The function returns the number of possible completions (without the terminating
+NULL value).
+
+Behind the scene
+----------------
 
 The structure created is named with a special prefix and placed by
 the linker in a special section using the linker lists mechanism