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

[Heinrich Schuchardt]

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The device-tree built into U-Boot is often out of date and not usable to 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.

Best regards

Heinrich