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

[Heinrich Schuchardt]

On 10/22/23 19:08, Tom Rini wrote:
> On Sun, Oct 22, 2023 at 06:34:08PM +0200, Heinrich Schuchardt wrote:
>> On 10/22/23 17:55, Tom Rini wrote:
>>> 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.
>>
>> Since Linux v6.5. That is why I wrote "currently". Once we migrate the
>> values of $fdtfile in U-Boot we may want to change
>> distro_efi_try_bootflow_files() to search both with and without vendor
>> directory.
>>
>> Are there already plans for that migration?
> 
> Right, v6.5 is out and has this change and v6.6 will be out soon enough,
> so the documentation we're adding here and now should be worded such
> that doesn't get stuck on these specifics.

Should I add a sentence:

Linux v6.5 has added vendor directories on 32bit ARM and U-Boot is 
expected to follow suit.

Best regards

Heinrich

> 
> And then how this kind of breaking change will be handled by
> distros and everyone else is something I wonder about.
>