[PATCH v3 1/1] efi_loader: expose the device-tree file name

[Tom Rini]

On Sun, Oct 22, 2023 at 10:47:33AM +0200, Heinrich Schuchardt wrote:

> Forward and backward compatibility of Linux kernel device-trees is
> sometimes missing. One solution approach is to load a kernel specific
> device-tree. This can either be done via a U-Boot scripts (like the one
> generated by Debian package flash-kernel or by a boot loader like GRUB.
> The boot loader approach currently requires to know the device-tree name
> before first boot which makes it unusable for generic images.
> 
> Expose the device-tree file name as EFI variable FdtFile.
> This will allow bootloaders to load a kernel specific device-tree.
> 
> The variable will not be exposed on ACPI based systems or if the
> environment variable fdtfile is not defined.
> 
> Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt at canonical.com>
> ---
> v3:
> 	Add documentation
> v2:
> 	Use a unique GUID to enable future U-Boot independent
> 	standardization.
> 	Do not try to add the variable on ACPI based systems.
> ---
>  doc/develop/uefi/uefi.rst  | 17 +++++++++++++++++
>  include/efi_loader.h       |  5 +++++
>  lib/efi_loader/efi_setup.c | 30 ++++++++++++++++++++++++++++++
>  3 files changed, 52 insertions(+)
> 
> diff --git a/doc/develop/uefi/uefi.rst b/doc/develop/uefi/uefi.rst
> index fb16ac743a..a81d09ae81 100644
> --- a/doc/develop/uefi/uefi.rst
> +++ b/doc/develop/uefi/uefi.rst
> @@ -916,6 +916,23 @@ So our final format of the FilePathList[] is::
>  
>      Loaded image - end node (0xff) - VenMedia - initrd_1 - [end node (0x01) - initrd_n ...] - end node (0xff)
>  
> +EFI variable FdtFile
> +~~~~~~~~~~~~~~~~~~~~
> +
> +Ideally U-Boot would always expose a device-tree that can be used for booting
> +any operating systems. Unfortunately operating systems like Linux sometimes
> +break forward and backward compatibility. In this case there is a need to load
> +an operating system version specific device-tree.
> +
> +U-Boot has an environment variable fdtfile identifying the device-tree file to
> +load. The content of this variable is exposed as EFI variable Fdtfile, vendor
> +GUID d45dde69-3bd6-40e0-90d5-6b606aa57730. It contains the device-tree path
> +name as a NUL terminated ASCII string.
> +
> +On 32bit ARM this is currently only a file name, e.g. 'imx6dl-wandboard.dtb'.
> +On other architectures the file name is preceded by the vendor directory, e.g.
> +'rockchip/rk3326-odroid-go2.dtb'.

The Linux Kernel has split 32bit ARM up by directory now, too.

-- 
Tom
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