7d92da0eef
Add some information about this new bootmeth, including how to enable it and how it works. Series-to: concept Cover-letter: boot: Enhance VBE to support a separate devicetree FIT It is sometimes desirable to have the devicetrees packaged with firware, or in a different FIT from the OS. This series adds support for this, including a test. The new bootmeth can automatically locate a state file and use that to decide what images to use when booting. The OS must update the file before rebooting, if a different selection is required. For now there is no logic to deal with boot failures. END Signed-off-by: Simon Glass <sjg@chromium.org>
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2.3 KiB
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62 lines
2.3 KiB
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.. SPDX-License-Identifier: GPL-2.0+:
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VBE OS Bootmeth
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===============
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VBE supports a separate FIT for devicetrees and the OS. This bootmeth detects
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the `vbe-state` file on the media and uses this to determine the bootflow to
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use: there may be two or three separate FITs, for the A, B and recovery flows.
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In embedded systems it is common to package the devicetrees with the OS in
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a FIT. Thus both can be updated together by writing a single file.
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For distros which want to provide broad hardware support, it is sometimes
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desirable to package the devicetrees separately from the OS, so they can have
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their own OEM-controlled lifecycle. This allows the OS package to be somewhat
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smalller than if it had all the devicetrees included.
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In terms of lifecycle, the OEM may update the devicetree FIT as part of firmware
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update, or a separate OS package.
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There are challenges with this approach and it is best suited for fairly stable
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platforms. Particularly in the early days of Linux supporting a particular
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SoC, the devicetree/kernel interface may not be fully stable. It is often more
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desirable to transition to using an OEM FIT once stability is achieved.
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It is also possible to put small overlays in the OS FIT, e.g. to enable a
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feature which was not working at launch. This results in smaller FITs tha
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packaging the entire devicetree for each board.
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The compatible string "vbe,abrec-os" is used for the driver. It is present
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if `CONFIG_BOOTMETH_VBE_ABREC_OS` is enabled.
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For more information on VBE, see :doc:`../vbe`.
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Partition format
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----------------
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Boot files should be stored on a separate boot partition, formatted as ext4.
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Each slot (A, B, recovery) has a subdirectory containing an
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`extlinux/extlinux.conf` file. This provides information about the OS fit for
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each OS version.
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The VBE bootmeth requires a file called `vbe-state` to be present in the root
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directory of the boot partition, to indicate which slot to boot.
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The `vbe-state` file is a devicetree blob, with the following schema:
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root node
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compatible: "vbe,abrec-state"
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/os node:
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compatible: "vbe,os-state"
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/os/next-boot:
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Indicates the slot that will be used on the next boot. It has a single
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property:
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slot (string)
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Indicates the slot that will be used on the next boot; valid values
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are "a", "b", and "recovery"
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