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boot-loader-spec: add devicetree-overlay key

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Device tree overlays are a convenient way to patch device trees, e.g.,
add new devices to a device tree or enable/disable devices. This is
useful for non-discoverable but configurable hardware. Device tree
overlays are commonly used for displays on the Raspberry Pi or for
describing the content of FPGA bitstreams.

Add the devicetree-overlay key to boot loader specification entries to
allow boot loaders to apply overlays.

See #13537
This commit is contained in:
Michael Tretter 2019-10-18 15:14:35 +02:00 committed by Zbigniew Jędrzejewski-Szmek
parent 228ae7ff11
commit 09ee387e08
1 changed files with 6 additions and 0 deletions
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docs/BOOT_LOADER_SPECIFICATION.md
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@ -112,6 +112,12 @@ These configuration snippets shall be Unix-style text files (i.e. line separatio
* `devicetree` refers to the binary device tree to use when executing the
kernel. This also shall be a path relative to the `$BOOT` directory. This
key is optional. Example: `6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.armv7hl/tegra20-paz00.dtb`.
* `devicetree-overlay` refers to a list of device tree overlays that should be
applied by the boot loader. Multiple overlays are separated by spaces and
applied in the same order as they are listed. This key is optional but depends
on the `devicetree` key. Example:
`/6a9857a393724b7a981ebb5b8495b9ea/overlays/overlay_A.dtbo
/6a9857a393724b7a981ebb5b8495b9ea/overlays/overlay_B.dtbo`
* `architecture` refers to the architecture this entry is defined for. The argument should be an architecture identifier, using the architecture vocabulary defined by the EFI specification (i.e. `IA32`, `x64`, `IA64`, `ARM`, `AA64`, …). If specified and this does not match (case insensitively) the local system architecture this entry should be hidden.
Each configuration drop-in snippet must include at least a `linux` or an `efi` key and is otherwise not valid. Here's an example for a complete drop-in file: