doc: Add a description for bootmeth_pxe

Add documentation for the pxe bootmeth.

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

doc/develop/bootstd/extlinux.rst

@@ -14,7 +14,8 @@ 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.
+Note that the :doc:`pxelinux` 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.

doc/develop/bootstd/index.rst

@@ -8,3 +8,4 @@ Standard Boot
 
    overview
    extlinux
+   pxelinux

doc/develop/bootstd/overview.rst

@@ -417,7 +417,7 @@ Available bootmeth drivers
 Bootmeth drivers are provided for booting from various media:
 
    - :doc:`extlinux / syslinux <extlinux>` boot from a storage device
-   - extlinux boot from a network (PXE)
+   - :doc:`extlinux / syslinux <extlinux>` boot from a network (PXE)
    - U-Boot scripts from disk, network or SPI flash
    - EFI boot using bootefi from disk
    - VBE

doc/develop/bootstd/pxelinux.rst

@@ -0,0 +1,27 @@
+.. SPDX-License-Identifier: GPL-2.0+:
+
+PXE Bootmeth
+============
+
+PXE (Preboot eXecution-Environment) provides a way to boot an operating system
+over a network interface. The PXE bootmeth supports PXELINUX and 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 described
+`here <https://uapi-group.org/specifications/specs/boot_loader_specification>`_.
+It consists primarily of a list of named operating systems along with the
+kernel, initial ramdisk and other settings. The file is retrieved from a network
+server using the TFTP protocol.
+
+When invoked on a bootdev, this bootmeth searches for the file and creates a
+bootflow if found. See
+`PXELINUX <https://wiki.syslinux.org/wiki/index.php?title=PXELINUX>`_ for
+a full description of the search procedure.
+
+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-pxe" is used for the driver. It is
+present if `CONFIG_BOOTMETH_EXTLINUX_PXE` is enabled.