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

[Ilias Apalodimas]

Hi Heinrich,

On Tue, 17 Oct 2023 at 13:12, Heinrich Schuchardt <
heinrich.schuchardt at canonical.com> wrote:

> On 17.10.23 11:49, Ilias Apalodimas wrote:
> >
> >
> > On Tue, 17 Oct 2023 at 12:23, Heinrich Schuchardt
> > <heinrich.schuchardt at canonical.com
> > <mailto: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>
> >      > <mailto: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>
> >      >     <mailto: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?
>
> No, I meant read. It is U-Boot writing the variable.
>
> Concerning the GUID the UEFI 2.10 specification requires a unique
> VendorGuid other than EFI_GLOBAL_VARIABLE.
>
> If we think about adding a recommendation to the EBBR, it might make
> sense to use a GUID other than a U-Boot specific one (at the expense of
> 16 bytes of code).
>
> Best regards
>
> Heinrich
>

Ok, I like the idea of standardizing it, since GRUB needs to pick this up
eventually.
I guess we can use the u-boot GUID until is there, so

Reviewed-by: Ilias Apalodimas <ilias.apalodimas at linaro.org>