doc: Add a description for bootmeth_cros

Add documentation for the cros 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/cros.rst

@@ -0,0 +1,33 @@
+.. SPDX-License-Identifier: GPL-2.0+:
+
+ChromiumOS Bootmeth
+===================
+
+ChromiumOS provides a mechanism for booting its Operating System from a block
+device, described
+`here <https://www.chromium.org/chromium-os/chromiumos-design-docs/verified-boot/>`_.
+
+U-Boot includes support for reading the associated data structures from the
+device and identifying a bootable ChromiumOS image. This structure includes the
+kernel itself, boot arguments (kernel command line), as well as the x86 setup
+block (for x86 only).
+
+When invoked on a bootdev, this bootmeth searches for kernel partitions with
+the appropriate GUID (Globally Unique Identifier). When found, the information
+is loaded and a bootflow is created.
+
+When the bootflow is booted, the bootmeth reads the kernel and boot arguments.
+It then boots the kernel using zboot (on x86) or bootm (on ARM). The boot
+arguments are adjusted to replace `%U` with the UUID of the selected kernel
+partition. This results in the correct root disk being used, which is the next
+partition after the kernel partition.
+
+For ARM, a :doc:`/usage/fit/index` is used. The `CONFIG_FIT_BEST_MATCH` option
+must be enabled for U-Boot to select the correct devicetree to boot with.
+
+Note that a ChromiumOS image typically has two copies of the Operating System,
+each with its own kernel and root disk. There is no initial ramdisk (initrd).
+This means that this bootmeth typically locates two separate images.
+
+The compatible string "u-boot,cros" is used for the driver. It is present
+if `CONFIG_BOOTMETH_CROS` is enabled.

doc/develop/bootstd/index.rst

@@ -10,3 +10,4 @@ Standard Boot
    extlinux
    pxelinux
    qfw
+   cros

doc/develop/bootstd/overview.rst

@@ -421,6 +421,7 @@ Bootmeth drivers are provided for booting from various media:
    - U-Boot scripts from disk, network or SPI flash
    - EFI boot using bootefi from disk
    - VBE
+   - :doc:`ChromiumOS <cros>` ChromiumOS boot from a disk
    - EFI boot using boot manager
    - Android bootflow (boot image v4)
    - :doc:`QFW <qfw>`: QEMU firmware interface