Add documentation for the cros bootmeth. Signed-off-by: Simon Glass <s...@chromium.org> ---
doc/develop/bootstd/cros.rst | 33 ++++++++++++++++++++++++++++++++ doc/develop/bootstd/index.rst | 1 + doc/develop/bootstd/overview.rst | 1 + 3 files changed, 35 insertions(+) create mode 100644 doc/develop/bootstd/cros.rst diff --git a/doc/develop/bootstd/cros.rst b/doc/develop/bootstd/cros.rst new file mode 100644 index 00000000000..96f148837ac --- /dev/null +++ b/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 OS, 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. 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 f8fce7207ce..69fd3c2d2eb 100644 --- a/doc/develop/bootstd/index.rst +++ b/doc/develop/bootstd/index.rst @@ -10,3 +10,4 @@ Standard Boot extlinux pxelinux qfw + cros diff --git a/doc/develop/bootstd/overview.rst b/doc/develop/bootstd/overview.rst index bcc2c00c775..f12e93236a7 100644 --- a/doc/develop/bootstd/overview.rst +++ b/doc/develop/bootstd/overview.rst @@ -417,6 +417,7 @@ Bootmeth drivers are provided for: - 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 - :doc:`QFW <qfw>`: QEMU firmware interface -- 2.34.1