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56418511e9
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>
34 lines
1.5 KiB
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34 lines
1.5 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0+:
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ChromiumOS Bootmeth
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===================
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ChromiumOS provides a mechanism for booting its Operating System from a block
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device, described
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`here <https://www.chromium.org/chromium-os/chromiumos-design-docs/verified-boot/>`_.
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U-Boot includes support for reading the associated data structures from the
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device and identifying a bootable ChromiumOS image. This structure includes the
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kernel itself, boot arguments (kernel command line), as well as the x86 setup
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block (for x86 only).
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When invoked on a bootdev, this bootmeth searches for kernel partitions with
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the appropriate GUID (Globally Unique Identifier). When found, the information
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is loaded and a bootflow is created.
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When the bootflow is booted, the bootmeth reads the kernel and boot arguments.
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It then boots the kernel using zboot (on x86) or bootm (on ARM). The boot
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arguments are adjusted to replace `%U` with the UUID of the selected kernel
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partition. This results in the correct root disk being used, which is the next
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partition after the kernel partition.
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For ARM, a :doc:`/usage/fit/index` is used. The `CONFIG_FIT_BEST_MATCH` option
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must be enabled for U-Boot to select the correct devicetree to boot with.
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Note that a ChromiumOS image typically has two copies of the Operating System,
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each with its own kernel and root disk. There is no initial ramdisk (initrd).
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This means that this bootmeth typically locates two separate images.
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The compatible string "u-boot,cros" is used for the driver. It is present
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if `CONFIG_BOOTMETH_CROS` is enabled.
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