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

[Mark Kettenis]

> From: Simon Glass <sjg at google.com>
> Date: Thu, 19 Oct 2023 07:55:49 -0600
> 
> 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 The device-tree built into U-Boot is often out of date and not usable to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.

>From an OpenBSD perspective:

* It allows us to load directly from an OpenBSD filesystem.  (And yes
  I really think you don't want to add support for all the different
  flavours of the UFS/FFS filesystem to U-Boot).

* It means we don't have to worry about breaking other OS installs on
  the same disk since we can avoid copying the device tree onto the
  (shared) EFI System Partition.

* It means that we can provide a uniform user interface that is
  independent on the UEFI implementation; the user doesn't need to
  care whether they are dealing with U-Boot, EDK2 or some other
  proprietary UEFI implementation.

Cheers,

Mark