On Tue, 24 Oct 2023 at 09:20, Heinrich Schuchardt
<heinrich.schucha...@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.
>
> 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.schucha...@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.
> +
> +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 many architectures the file name is
> +preceded by a vendor directory ('vendor-directory/board-name.dtb').
> +
> Links
> -----
>
> diff --git a/include/efi_loader.h b/include/efi_loader.h
> index e24410505f..146e7f1bce 100644
> --- a/include/efi_loader.h
> +++ b/include/efi_loader.h
> @@ -152,6 +152,11 @@ static inline efi_status_t efi_launch_capsules(void)
> EFI_GUID(0x8108ac4e, 0x9f11, 0x4d59, \
> 0x85, 0x0e, 0xe2, 0x1a, 0x52, 0x2c, 0x59, 0xb2)
>
> +/* Vendor GUID for the FdtFile variable */
> +#define VENDOR_FDTFILE_GUID \
> + EFI_GUID(0xd45dde69, 0x3bd6, 0x40e0, \
> + 0x90, 0xd5, 0x6b, 0x60, 0x6a, 0xa5, 0x77, 0x30)
> +
> /* Use internal device tree when starting UEFI application */
> #define EFI_FDT_USE_INTERNAL NULL
>
> diff --git a/lib/efi_loader/efi_setup.c b/lib/efi_loader/efi_setup.c
> index e6de685e87..71bcde645b 100644
> --- a/lib/efi_loader/efi_setup.c
> +++ b/lib/efi_loader/efi_setup.c
> @@ -17,6 +17,8 @@
>
> efi_status_t efi_obj_list_initialized = OBJ_LIST_NOT_INITIALIZED;
>
> +efi_guid_t vendor_fdtfile_guid = VENDOR_FDTFILE_GUID;
> +
> /*
> * Allow unaligned memory access.
> *
> @@ -26,6 +28,27 @@ 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",
> + &vendor_fdtfile_guid,
> + EFI_VARIABLE_BOOTSERVICE_ACCESS |
> + EFI_VARIABLE_RUNTIME_ACCESS |
> + EFI_VARIABLE_READ_ONLY,
> + strlen(val) + 1, val, false);
> +}
> +
> /**
> * efi_init_platform_lang() - define supported languages
> *
> @@ -250,6 +273,13 @@ efi_status_t efi_init_obj_list(void)
> if (ret != EFI_SUCCESS)
> goto out;
>
> + /* Define EFI variable FdtFile */
> + if (!CONFIG_IS_ENABLED(GENERATE_ACPI_TABLE)) {
> + ret = efi_init_fdtfile();
> + if (ret != EFI_SUCCESS)
> + goto out;
> + }
> +
> /* Indicate supported features */
> ret = efi_init_os_indications();
> if (ret != EFI_SUCCESS)
> --
> 2.40.1
>
Reviewed-by: Ilias Apalodimas <ilias.apalodi...@linaro.org>
