1UEFI, the Unified Extensible Firmware Interface, is a specification
2governing the behaviours of compatible firmware interfaces. It is
3maintained by the UEFI Forum - http://www.uefi.org/.
4
5UEFI is an evolution of its predecessor 'EFI', so the terms EFI and
6UEFI are used somewhat interchangeably in this document and associated
7source code. As a rule, anything new uses 'UEFI', whereas 'EFI' refers
8to legacy code or specifications.
9
10UEFI support in Linux
11=====================
12Booting on a platform with firmware compliant with the UEFI specification
13makes it possible for the kernel to support additional features:
14- UEFI Runtime Services
15- Retrieving various configuration information through the standardised
16  interface of UEFI configuration tables. (ACPI, SMBIOS, ...)
17
18For actually enabling [U]EFI support, enable:
19- CONFIG_EFI=y
20- CONFIG_EFI_VARS=y or m
21
22The implementation depends on receiving information about the UEFI environment
23in a Flattened Device Tree (FDT) - so is only available with CONFIG_OF.
24
25UEFI stub
26=========
27The "stub" is a feature that extends the Image/zImage into a valid UEFI
28PE/COFF executable, including a loader application that makes it possible to
29load the kernel directly from the UEFI shell, boot menu, or one of the
30lightweight bootloaders like Gummiboot or rEFInd.
31
32The kernel image built with stub support remains a valid kernel image for
33booting in non-UEFI environments.
34
35UEFI kernel support on ARM
36==========================
37UEFI kernel support on the ARM architectures (arm and arm64) is only available
38when boot is performed through the stub.
39
40When booting in UEFI mode, the stub deletes any memory nodes from a provided DT.
41Instead, the kernel reads the UEFI memory map.
42
43The stub populates the FDT /chosen node with (and the kernel scans for) the
44following parameters:
45________________________________________________________________________________
46Name                      | Size   | Description
47================================================================================
48linux,uefi-system-table   | 64-bit | Physical address of the UEFI System Table.
49--------------------------------------------------------------------------------
50linux,uefi-mmap-start     | 64-bit | Physical address of the UEFI memory map,
51                          |        | populated by the UEFI GetMemoryMap() call.
52--------------------------------------------------------------------------------
53linux,uefi-mmap-size      | 32-bit | Size in bytes of the UEFI memory map
54                          |        | pointed to in previous entry.
55--------------------------------------------------------------------------------
56linux,uefi-mmap-desc-size | 32-bit | Size in bytes of each entry in the UEFI
57                          |        | memory map.
58--------------------------------------------------------------------------------
59linux,uefi-mmap-desc-ver  | 32-bit | Version of the mmap descriptor format.
60--------------------------------------------------------------------------------
61linux,uefi-stub-kern-ver  | string | Copy of linux_banner from build.
62--------------------------------------------------------------------------------
63
64For verbose debug messages, specify 'uefi_debug' on the kernel command line.
65