doc: arm: add UEFI support documentation

This patch provides documentation of the [U]EFI runtime service and
configuration features for the arm architecture.

Cc: Rob Landley <rob@landley.net>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
Acked-by: Grant Likely <grant.likely@linaro.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>

Documentation/arm/00-INDEX

@@ -46,5 +46,7 @@ swp_emulation
 	- SWP/SWPB emulation handler/logging description
 tcm.txt
 	- ARM Tightly Coupled Memory
+uefi.txt
+	- [U]EFI configuration and runtime services documentation
 vlocks.txt
 	- Voting locks, low-level mechanism relying on memory system atomic writes.

Documentation/arm/uefi.txt

+UEFI, the Unified Extensible Firmware Interface, is a specification
+governing the behaviours of compatible firmware interfaces. It is
+maintained by the UEFI Forum - http://www.uefi.org/.
+
+UEFI is an evolution of its predecessor 'EFI', so the terms EFI and
+UEFI are used somewhat interchangeably in this document and associated
+source code. As a rule, anything new uses 'UEFI', whereas 'EFI' refers
+to legacy code or specifications.
+
+UEFI support in Linux
+=====================
+Booting on a platform with firmware compliant with the UEFI specification
+makes it possible for the kernel to support additional features:
+- UEFI Runtime Services
+- Retrieving various configuration information through the standardised
+  interface of UEFI configuration tables. (ACPI, SMBIOS, ...)
+
+For actually enabling [U]EFI support, enable:
+- CONFIG_EFI=y
+- CONFIG_EFI_VARS=y or m
+
+The implementation depends on receiving information about the UEFI environment
+in a Flattened Device Tree (FDT) - so is only available with CONFIG_OF.
+
+UEFI stub
+=========
+The "stub" is a feature that extends the Image/zImage into a valid UEFI
+PE/COFF executable, including a loader application that makes it possible to
+load the kernel directly from the UEFI shell, boot menu, or one of the
+lightweight bootloaders like Gummiboot or rEFInd.
+
+The kernel image built with stub support remains a valid kernel image for
+booting in non-UEFI environments.
+
+UEFI kernel support on ARM
+==========================
+UEFI kernel support on the ARM architectures (arm and arm64) is only available
+when boot is performed through the stub.
+
+When booting in UEFI mode, the stub deletes any memory nodes from a provided DT.
+Instead, the kernel reads the UEFI memory map.
+
+The stub populates the FDT /chosen node with (and the kernel scans for) the
+following parameters:
+________________________________________________________________________________
+Name                      | Size   | Description
+================================================================================
+linux,uefi-system-table   | 64-bit | Physical address of the UEFI System Table.
+--------------------------------------------------------------------------------
+linux,uefi-mmap-start     | 64-bit | Physical address of the UEFI memory map,
+                          |        | populated by the UEFI GetMemoryMap() call.
+--------------------------------------------------------------------------------
+linux,uefi-mmap-size      | 32-bit | Size in bytes of the UEFI memory map
+                          |        | pointed to in previous entry.
+--------------------------------------------------------------------------------
+linux,uefi-mmap-desc-size | 32-bit | Size in bytes of each entry in the UEFI
+                          |        | memory map.
+--------------------------------------------------------------------------------
+linux,uefi-mmap-desc-ver  | 32-bit | Version of the mmap descriptor format.
+--------------------------------------------------------------------------------
+linux,uefi-stub-kern-ver  | string | Copy of linux_banner from build.
+--------------------------------------------------------------------------------
+
+For verbose debug messages, specify 'uefi_debug' on the kernel command line.