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

[Simon Glass]

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 The device-tree built into U-Boot is often out of date and not usable to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

Also, can we test this interface?

Regards,
Simon