doc: describe load command

Man-page for load command.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>

doc/usage/index.rst

@@ -23,6 +23,7 @@ Shell commands
    exit
    false
    for
+   load
    loady
    mbr
    pstore

doc/usage/load.rst

@@ -0,0 +1,74 @@
+.. SPDX-License-Identifier: GPL-2.0+:
+
+load command
+============
+
+Synopsis
+--------
+
+::
+
+    load <interface> [<dev[:part]> [<addr> [<filename> [bytes [pos]]]]]
+
+Description
+-----------
+
+The load command is used to read a file from a filesystem into memory.
+
+The number of transferred bytes is saved in the environment variable filesize.
+The load address is saved in the environment variable fileaddr.
+
+interface
+    interface for accessing the block device (mmc, sata, scsi, usb, ....)
+
+dev
+    device number
+
+part
+    partition number, defaults to 0 (whole device)
+
+addr
+    load address, defaults to environment variable loadaddr or if loadaddr is
+    not set to configuration variable CONFIG_SYS_LOAD_ADDR
+
+filename
+    path to file, defaults to environment variable bootfile
+
+bytes
+    maximum number of bytes to load
+
+pos
+    number of bytes to skip
+
+addr, bytes, pos are hexadecimal numbers.
+
+Example
+-------
+
+::
+
+    => load mmc 0:1 ${kernel_addr_r} snp.efi
+    149280 bytes read in 11 ms (12.9 MiB/s)
+    =>
+    => load mmc 0:1 ${kernel_addr_r} snp.efi 1000000
+    149280 bytes read in 9 ms (15.8 MiB/s)
+    =>
+    => load mmc 0:1 ${kernel_addr_r} snp.efi 1000000 100
+    149024 bytes read in 10 ms (14.2 MiB/s)
+    =>
+    => load mmc 0:1 ${kernel_addr_r} snp.efi 10
+    16 bytes read in 1 ms (15.6 KiB/s)
+    =>
+
+Configuration
+-------------
+
+The load command is only available if CONFIG_CMD_FS_GENERIC=y.
+
+Return value
+------------
+
+The return value $? is set to 0 (true) if the file was successfully loaded
+even if the number of bytes is less then the specified length.
+
+If an error occurs, the return value $? is set to 1 (false).