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

[Heinrich Schuchardt]

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.

Best regards

Heinrich


> 
> [...]
> 
> 
> Thanks
> /Ilias