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>
---
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'.
+
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
