[PATCH v3 1/1] efi_loader: expose the device-tree file name

[Tom Rini]

On Sun, Oct 22, 2023 at 06:34:08PM +0200, Heinrich Schuchardt wrote:
> On 10/22/23 17:55, Tom Rini wrote:
> > On Sun, Oct 22, 2023 at 10:47:33AM +0200, Heinrich Schuchardt 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>
> > > ---
> > > 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  | 17 +++++++++++++++++
> > >   include/efi_loader.h       |  5 +++++
> > >   lib/efi_loader/efi_setup.c | 30 ++++++++++++++++++++++++++++++
> > >   3 files changed, 52 insertions(+)
> > > 
> > > diff --git a/doc/develop/uefi/uefi.rst b/doc/develop/uefi/uefi.rst
> > > index fb16ac743a..a81d09ae81 100644
> > > --- a/doc/develop/uefi/uefi.rst
> > > +++ b/doc/develop/uefi/uefi.rst
> > > @@ -916,6 +916,23 @@ 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.
> > > +
> > > +U-Boot has an environment variable fdtfile identifying the device-tree file to
> > > +load. The content of this variable is exposed as EFI variable Fdtfile, vendor
> > > +GUID d45dde69-3bd6-40e0-90d5-6b606aa57730. It contains the device-tree path
> > > +name as a NUL terminated ASCII string.
> > > +
> > > +On 32bit ARM this is currently only a file name, e.g. 'imx6dl-wandboard.dtb'.
> > > +On other architectures the file name is preceded by the vendor directory, e.g.
> > > +'rockchip/rk3326-odroid-go2.dtb'.
> > 
> > The Linux Kernel has split 32bit ARM up by directory now, too.
> 
> Since Linux v6.5. That is why I wrote "currently". Once we migrate the
> values of $fdtfile in U-Boot we may want to change
> distro_efi_try_bootflow_files() to search both with and without vendor
> directory.
> 
> Are there already plans for that migration?

Right, v6.5 is out and has this change and v6.6 will be out soon enough,
so the documentation we're adding here and now should be worded such
that doesn't get stuck on these specifics.

And then how this kind of breaking change will be handled by
distros and everyone else is something I wonder about.

-- 
Tom
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