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Commit e1977464 authored by Leif Lindholm's avatar Leif Lindholm Committed by Matt Fleming
Browse files

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: default avatarLeif Lindholm <leif.lindholm@linaro.org>
Acked-by: default avatarGrant Likely <grant.likely@linaro.org>
Acked-by: default avatarCatalin Marinas <catalin.marinas@arm.com>
Signed-off-by: default avatarMatt Fleming <matt.fleming@intel.com>
parent f84d0275

Documentation/arm/00-INDEX

+2 −0
Original line number Diff line number Diff line
@@ -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

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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.