doc: Add a description for bootmeth_script

Add documentation for the script bootmeth.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>

3 files changed, 54 insertions, 1 deletions

diff --git a/doc/develop/bootstd/index.rst b/doc/develop/bootstd/index.rst
index 2c5d2d3fade..9d35b567d55 100644
--- a/doc/develop/bootstd/index.rst
+++ b/doc/develop/bootstd/index.rst
@@ -11,4 +11,5 @@ Standard Boot
    pxelinux
    qfw
    cros
+   script
    sandbox
diff --git a/doc/develop/bootstd/overview.rst b/doc/develop/bootstd/overview.rst
index 20c5d56aad2..bc7e4b39704 100644
--- a/doc/develop/bootstd/overview.rst
+++ b/doc/develop/bootstd/overview.rst
@@ -418,7 +418,7 @@ Bootmeth drivers are provided for booting from various media:
 
    - :doc:`extlinux / syslinux <extlinux>` boot from a storage device
    - :doc:`extlinux / syslinux <extlinux>` boot from a network (PXE)
-   - U-Boot scripts from disk, network or SPI flash
+   - :doc:`U-Boot scripts <script>` from disk, network or SPI flash
    - EFI boot using bootefi from disk
    - VBE
    - :doc:`ChromiumOS <cros>` ChromiumOS boot from a disk
diff --git a/doc/develop/bootstd/script.rst b/doc/develop/bootstd/script.rst
new file mode 100644
index 00000000000..47f3684b86b
--- /dev/null
+++ b/doc/develop/bootstd/script.rst
@@ -0,0 +1,52 @@
+.. SPDX-License-Identifier: GPL-2.0+:
+
+Script Bootmeth
+===============
+
+This bootmeth provides a way to locate and run a script on a block or network
+device. It can also support SPI flash.
+
+For a block device the file is read from the selected partition, which must use
+a supported filesystem. The subdirectory to search in is defined by the bootstd
+list of prefixes (``{"/", "/boot"}`` by default) and can be adjust with the
+`filename-prefixes` property in the bootstd device.
+
+For a network device, the filename is obtained from the `boot_script_dhcp`
+environment variable and the file is read using tftp. It must be in the
+top-level directory of the tftp server.
+
+In either case (file or network), the bootmeth searches for the file and creates
+a bootflow if found. The bootmeth searches for "boot.scr.uimg" first, then
+"boot.scr" if not found.
+
+For SPI flash, a script is read from flash using the offset provided by the
+"script_offset_f" environment variable.
+
+Some attempt is made to identify the Operating System: so far this only detects
+an `Armbian <https://www.armbian.com>`_
+distro. For block devices, if a file called "boot.bmp" exists in the same
+directory then it is used as the bootflow logo.
+
+When the bootflow is booted, the bootmeth sets these environment variables:
+
+    devtype
+        device type (e.g. "usb", "mmc", "ethernet" or "spi_flash")
+
+    devnum
+        device number, corresponding to the device 'sequence' number
+        ``dev_seq(dev)``
+
+    distro_bootpart
+        (block devices only) partition number on the device (numbered from 1)
+
+    prefix
+        prefix used to find the file
+
+    mmc_bootdev
+        device number (same as `devnum`), set for sunxi mmc devices only
+
+The script file must be a FIT or a legacy uImage. It is loaded into memory and
+executed.
+
+The compatible string "u-boot,script" is used for the driver. It is present
+if `CONFIG_BOOTMETH_SCRIPT` is enabled.