[PATCH 03/13] doc: Add a description for bootmeth_extlinux

[Simon Glass <sjg@chromium.org>]

Add documentation for the extlinux bootmeth.

Signed-off-by: Simon Glass <sjg@chromium.org>
---

 doc/develop/bootstd/extlinux.rst | 27 +++++++++++++++++++++++++++
 doc/develop/bootstd/index.rst    |  1 +
 doc/develop/bootstd/overview.rst |  2 +-
 3 files changed, 29 insertions(+), 1 deletion(-)
 create mode 100644 doc/develop/bootstd/extlinux.rst

diff --git a/doc/develop/bootstd/extlinux.rst b/doc/develop/bootstd/extlinux.rst
new file mode 100644
index 00000000000..28490f38899
--- /dev/null
+++ b/doc/develop/bootstd/extlinux.rst
@@ -0,0 +1,27 @@
+.. SPDX-License-Identifier: GPL-2.0+:
+
+Extlinux Bootmeth
+=================
+
+`Extlinux <https://uapi-group.org/specifications/specs/boot_loader_specification>`_
+(sometimes called syslinux) allows U-Boot to provide a menu of possible
+Operating Systems from which the user can choose.
+
+U-Boot includes a parser for the `extlinux.conf` file. It consists primarily of
+a list of named OSes along with the kernel, initial ramdisk and other settings.
+The file is stored in the `extlinux/` subdirectory, possibly under the `boot/`
+subdirectory. This list of prefixes ({"/", "/boot"} by default) can be selected
+with the `filename-prefixes` property in the bootstd device.
+
+Note that PXE (Preboot eXecution-Environment) uses the same file format, but in
+a network context.
+
+When invoked on a bootdev, this bootmeth searches for the file and creates a
+bootflow if found.
+
+When the bootflow is booted, the bootmeth calls pxe_setup_ctx() to set up the
+context, then pxe_process() to process the file. Depending on the contents, this
+may boot an OS or provide a list of options to the user, perhaps with a timeout.
+
+The compatible string "u-boot,extlinux" is used for the driver. The driver is
+automatically instantiated if there are no bootmeth drivers in the devicetree.
diff --git a/doc/develop/bootstd/index.rst b/doc/develop/bootstd/index.rst
index f4f87c7787c..5bbb3d633a3 100644
--- a/doc/develop/bootstd/index.rst
+++ b/doc/develop/bootstd/index.rst
@@ -7,3 +7,4 @@ Standard Boot
    :maxdepth: 2
 
    overview
+   extlinux
diff --git a/doc/develop/bootstd/overview.rst b/doc/develop/bootstd/overview.rst
index 761f61a573b..086a0b1281d 100644
--- a/doc/develop/bootstd/overview.rst
+++ b/doc/develop/bootstd/overview.rst
@@ -412,7 +412,7 @@ Available bootmeth drivers
 
 Bootmeth drivers are provided for:
 
-   - extlinux / syslinux boot from a disk
+   - :doc:`extlinux / syslinux <extlinux>` boot from a disk
    - extlinux boot from a network (PXE)
    - U-Boot scripts from disk, network or SPI flash
    - EFI boot using bootefi from disk
-- 
2.34.1