[PATCH v2 1/1] efi_loader: expose the device-tree file name

[Heinrich Schuchardt]

On 19.10.23 15:55, Simon Glass wrote:
> Hi Heinrich,
> 
> On Wed, 18 Oct 2023 at 02:15, Heinrich Schuchardt
> <heinrich.schuchardt at canonical.com> wrote:
>>
>> On 10/18/23 05:33, Simon Glass wrote:
>>> Hi Heinrich,
>>>
>>> On Tue, 17 Oct 2023 at 07:50, Heinrich Schuchardt
>>> <heinrich.schuchardt at canonical.com> 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>
>>>> ---
>>>> v2:
>>>>           Use a unique GUID to enable future U-Boot independent
>>>>           standardization.
>>>>           Do not try to add the variable on ACPI based systems.
>>>> ---
>>>>    include/efi_loader.h       |  5 +++++
>>>>    lib/efi_loader/efi_setup.c | 30 ++++++++++++++++++++++++++++++
>>>>    2 files changed, 35 insertions(+)
>>>
>>> I was too slow to reply to v1.
>>>
>>> Does grub load the DT? I was assuming that U-Boot would pass it on?
>>> What is the interface between U-Boot and grub?
>>
>> The device-tree built into U-Boot is often out of date and not usable to
>> boot current Linux. A single device-tree can be loaded by U-Boot from
>> file and passed on as EFI configuration table. This device-tree may not
>> be compatible with all kernel versions exposed by GRUB.
>>
>> GRUB provides a devicetree command. It is disabled if you use secure
>> boot. At least in Debian and Ubuntu GRUB invokes the
>> EFI_DT_FIXUP_PROTOCOL exposed by U-Boot to run U-Boot's device-tree
>> fix-ups after loading a device-tree.
>>
>> Vendor scripts for GRUB like Ubuntu's /etc/grub.d/10_linux add
>> devicetree commands to the boot options in grub.cfg.
> 
> Thanks. I wonder if you could document this somewhere? It seems like
> there are a lot of options and it is quite complicated.
> 
> Back to the question, I suppose you are expecting grub to load the DT
> using this filename? But why doesn't U-Boot load it instead? It seems
> very convoluted.

A separate file of this name exists for every kernel version installed. 
The loaded dtb must match the kernel. U-Boot does not know what kernel 
version will be chosen in GRUB. And for a generic image GRUB does not 
what board it is on.

> 
> Also, can we test this interface?

Neither the sandbox nor QEMU have environment variable fdtfile. And we 
don't create the EFI variable with ACPI as used on the sandbox.

Best regards

Heinrich