[PATCH v3 04/14] doc: Add a description for bootmeth_extlinux

[Simon Glass]

Add documentation for the extlinux bootmeth.

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

Changes in v3:
- Use quoting on list and function names

Changes in v2:
- Adjustments from Heinrich's comments

 doc/develop/bootstd/extlinux.rst | 28 ++++++++++++++++++++++++++++
 doc/develop/bootstd/index.rst    |  1 +
 doc/develop/bootstd/overview.rst |  4 ++--
 3 files changed, 31 insertions(+), 2 deletions(-)
 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..46e9ad9eea1
--- /dev/null
+++ b/doc/develop/bootstd/extlinux.rst
@@ -0,0 +1,28 @@
+.. 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 available
+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 operating systems 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 the PXE bootmeth 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 operating system or provide a list of options to
+the user, perhaps with a timeout.
+
+The compatible string "u-boot,extlinux" is used for the driver. It is present
+if `CONFIG_BOOTMETH_EXTLINUX` is enabled.
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 d5051e986bb..7af159b9ca8 100644
--- a/doc/develop/bootstd/overview.rst
+++ b/doc/develop/bootstd/overview.rst
@@ -410,9 +410,9 @@ a good selection of boot options is available.
 Available bootmeth drivers
 --------------------------
 
-Bootmeth drivers are provided for:
+Bootmeth drivers are provided for booting from various media:
 
-   - extlinux / syslinux boot from a disk
+   - :doc:`extlinux / syslinux <extlinux>` boot from a storage device
    - extlinux boot from a network (PXE)
    - U-Boot scripts from disk, network or SPI flash
    - EFI boot using bootefi from disk
-- 
2.34.1