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

[Ilias Apalodimas]

On Tue, 17 Oct 2023 at 12:23, Heinrich Schuchardt <
heinrich.schuchardt at canonical.com> wrote:

> On 17.10.23 11:14, Ilias Apalodimas wrote:
> > Hi Heinrich,
> >
> > On Tue, 17 Oct 2023 at 11:55, Heinrich Schuchardt
> > <heinrich.schuchardt at canonical.com
> > <mailto: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.
> >
> >     Signed-off-by: Heinrich Schuchardt
> >     <heinrich.schuchardt at canonical.com
> >     <mailto:heinrich.schuchardt at canonical.com>>
> >     ---
> >       lib/efi_loader/efi_setup.c | 26 ++++++++++++++++++++++++++
> >       1 file changed, 26 insertions(+)
> >
> >     diff --git a/lib/efi_loader/efi_setup.c b/lib/efi_loader/efi_setup.c
> >     index e6de685e87..b24feb94dc 100644
> >     --- a/lib/efi_loader/efi_setup.c
> >     +++ b/lib/efi_loader/efi_setup.c
> >     @@ -26,6 +26,27 @@ void __weak allow_unaligned(void)
> >       {
> >       }
> >
> >     +/**
> >     + * efi_init_fdtfile() - set EFI variable FdtFile
> >     + *
> >     + * Return:     status code
> >     + */
> >     +static efi_status_t efi_init_fdtfile(void)
> >     +{
> >     +       char *val;
> >     +
> >     +       val = env_get("fdtfile");
> >     +       if (!val)
> >     +               return EFI_SUCCESS;
> >     +
> >     +       return efi_set_variable_int(u"FdtFile",
> >     +                                   &efi_u_boot_guid,
> >     +                                   EFI_VARIABLE_BOOTSERVICE_ACCESS |
> >     +                                   EFI_VARIABLE_RUNTIME_ACCESS |
> >
> >
> > Why is runtime access needed?  Wouldn't the choice be done before
> > ExitBootServices?
>
>
> Having runtime access will allow to use the content of the variable to
> write a devicetree command to grub.cfg or to copy the relevant
> device-tree to a predefined location.
>
> As GRUB cannot read EFI variables, yet, this would be the easiest
> short-term approach.
>
> Do you mean to write EFI variables?  If you want GRUB to be able to do
that, wouldn't it be better to use a generic GUID instead of the U-Boot one?

Regards
/Ilias

> Best regards
>
> Heinrich
>
>
> >
> > [...]
> >
> >
> > Thanks
> > /Ilias
>
>