[PATCH v4 1/1] efi_loader: expose the device-tree file name

[Tom Rini]

On Wed, Oct 25, 2023 at 10:28:05PM +0200, Mark Kettenis wrote:
> > Date: Wed, 25 Oct 2023 21:57:44 +0200
> > From: Heinrich Schuchardt <heinrich.schuchardt at canonical.com>
> > 
> > On 10/25/23 20:23, Simon Glass wrote:
> > > Hi Heinrich,
> > > 
> > > On Tue, 24 Oct 2023 at 18:02, Simon Glass <sjg at google.com> wrote:
> > >>
> > >> Hi Heinrich,
> > >>
> > >> On Mon, 23 Oct 2023 at 23:20, 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.
> > >>
> > >> kernel-specific
> > >>
> > >>>
> > >>> 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>
> > >>> ---
> > >>> v4:
> > >>>          Generalize the description of the content of $fdtfile.
> > >>> 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  | 14 ++++++++++++++
> > >>>   include/efi_loader.h       |  5 +++++
> > >>>   lib/efi_loader/efi_setup.c | 30 ++++++++++++++++++++++++++++++
> > >>>   3 files changed, 49 insertions(+)
> > >>>
> > >>> diff --git a/doc/develop/uefi/uefi.rst b/doc/develop/uefi/uefi.rst
> > >>> index fb16ac743a..702c490831 100644
> > >>> --- a/doc/develop/uefi/uefi.rst
> > >>> +++ b/doc/develop/uefi/uefi.rst
> > >>> @@ -916,6 +916,20 @@ 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.
> > >>
> > >> This seems to be a strong statement. Given the effort that goes into
> > >> the DT, changes are supposed to be backwards-compatible. Is this
> > >> generally true, or is it just that we want an up-to-date DT for the
> > >> kernel to enable new features?
> > > 
> > > Did you see this comment?
> > 
> > It would have been nice to put the person which made that comment on copy.
> > 
> > The truth lies in the world "supposed":
> > 
> > The idea of a device-tree that never needs to change is quite old and 
> > never became true on ARM devices.
> > 
> > We all know Linux tends to break both forward and backward compatibility 
> > of device-trees. Here is a nice example:
> > 
> > d0c6707ca423 ("arm64: dts: allwinner: H5: NanoPi Neo Plus2: phy-mode 
> > rgmii-id")
> > 
> > Driver changes broke forward and backwards compatibility of a lot of 
> > Allwinner boards.
> 
> Well, that happened in 2020.  Things have gotten better over time.

Well, yes and no.  Given the brief summary here, I bet this was just
like when phy-mode and am335x platforms had DT compatibility broken and
the answer was that it was OK because the DT was incorrectly describing
hardware. So this is the reminder that there are cases of breaking DT
compatibility that are allowed. Even if the DT has been out (and wrong)
for several years.

That's not the main point of this thread and I don't want to derail
things further along this point, I just want to note that the details
here reminded me of when things are allowed to be incompatible with
previous trees.

-- 
Tom
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