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Subject[PATCH v2 1/3] Documentation: arm: add UEFI support documentation
This patch provides documentation of the [U]EFI runtime services and
configuration features.

Signed-off-by: Leif Lindholm <>
Acked-by: Rob Landley <>
Documentation/arm/00-INDEX | 3 +++
Documentation/arm/uefi.txt | 47 ++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 50 insertions(+)
create mode 100644 Documentation/arm/uefi.txt

diff --git a/Documentation/arm/00-INDEX b/Documentation/arm/00-INDEX
index 4978456..87e01d1 100644
--- a/Documentation/arm/00-INDEX
+++ b/Documentation/arm/00-INDEX
@@ -36,3 +36,6 @@ nwfpe/
- NWFPE floating point emulator documentation
- SWP/SWPB emulation handler/logging description
+ - [U]EFI configuration and runtime services documentation
diff --git a/Documentation/arm/uefi.txt b/Documentation/arm/uefi.txt
new file mode 100644
index 0000000..e6e4d41
--- /dev/null
+++ b/Documentation/arm/uefi.txt
@@ -0,0 +1,47 @@
+UEFI, the Unified Extensible Firmware Interface is a speifcication
+governing the behaviours of compatible firmware interfaces. It is
+maintained by the UEFI Forum -
+Since UEFI is an evolution of its predecessor 'EFI', the terms EFI and
+UEFI are used somewhat interchangeably in this document and associated
+source code.
+The implementation depends on receiving the UEFI runtime memory map and a
+pointer to the System Table in a Flattened Device Tree - so is only available
+with CONFIG_OF.
+It parses the FDT /chosen node for the following parameters:
+- 'linux,efi-system-table':
+ Physical address of the system table. (required)
+ 64-bit value since an ARMv7 plattform may support LPAE, and to facilitate
+ code sharing with arm64. Top 32 bits will be ignored, since UEFI specification
+ mandates a 1:1 mapping of all RAM.
+- 'linux,efi-mmap':
+ The EFI memory map as an embedded property. (required)
+ An array of type EFI_MEMORY_DESCRIPTOR as described by the UEFI
+ specification, current version described in Linux by efi_memory_desc_t.
+ The memory map is represented in little-endian, not DT, byte order.
+ This map needs to contain at least the regions to be preserved for runtime
+ services, but would normally just be the map retreieved by calling UEFI
+ GetMemoryMap() immediately before ExitBootServices().
+- 'linux,efi-mmap-desc-size':
+ Size of each descriptor in the memory map. (override default)
+- 'linux,efi-mmap-desc-ver':
+ Memory descriptor format version. (override default)
+It also depends on early_memremap() to parse the UEFI configuration tables.
+For actually enabling [U]EFI support, enable:
+After the kernel has mapped the required regions into its address space,
+a SetVirtualAddressMap() call is made into UEFI in order to update
+relocations. This call must be performed with all the code in a 1:1
+mapping. This implementation achieves this by temporarily disabling the
+MMU for the duration of this call. This can only be done safely:
+- before secondary CPUs are brought online.
+- after early_initcalls have completed, since it uses setup_mm_for_reboot().
+For verbose debug messages, specify 'uefi_debug' on the kernel command

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