xref: /openbmc/linux/Documentation/arch/arm/uefi.rst (revision b4ba5e5e)
1================================================
2The Unified Extensible Firmware Interface (UEFI)
3================================================
4
5UEFI, the Unified Extensible Firmware Interface, is a specification
6governing the behaviours of compatible firmware interfaces. It is
7maintained by the UEFI Forum - http://www.uefi.org/.
8
9UEFI is an evolution of its predecessor 'EFI', so the terms EFI and
10UEFI are used somewhat interchangeably in this document and associated
11source code. As a rule, anything new uses 'UEFI', whereas 'EFI' refers
12to legacy code or specifications.
13
14UEFI support in Linux
15=====================
16Booting on a platform with firmware compliant with the UEFI specification
17makes it possible for the kernel to support additional features:
18
19- UEFI Runtime Services
20- Retrieving various configuration information through the standardised
21  interface of UEFI configuration tables. (ACPI, SMBIOS, ...)
22
23For actually enabling [U]EFI support, enable:
24
25- CONFIG_EFI=y
26- CONFIG_EFIVAR_FS=y or m
27
28The implementation depends on receiving information about the UEFI environment
29in a Flattened Device Tree (FDT) - so is only available with CONFIG_OF.
30
31UEFI stub
32=========
33The "stub" is a feature that extends the Image/zImage into a valid UEFI
34PE/COFF executable, including a loader application that makes it possible to
35load the kernel directly from the UEFI shell, boot menu, or one of the
36lightweight bootloaders like Gummiboot or rEFInd.
37
38The kernel image built with stub support remains a valid kernel image for
39booting in non-UEFI environments.
40
41UEFI kernel support on ARM
42==========================
43UEFI kernel support on the ARM architectures (arm and arm64) is only available
44when boot is performed through the stub.
45
46When booting in UEFI mode, the stub deletes any memory nodes from a provided DT.
47Instead, the kernel reads the UEFI memory map.
48
49The stub populates the FDT /chosen node with (and the kernel scans for) the
50following parameters:
51
52==========================  ======   ===========================================
53Name                        Type     Description
54==========================  ======   ===========================================
55linux,uefi-system-table     64-bit   Physical address of the UEFI System Table.
56
57linux,uefi-mmap-start       64-bit   Physical address of the UEFI memory map,
58                                     populated by the UEFI GetMemoryMap() call.
59
60linux,uefi-mmap-size        32-bit   Size in bytes of the UEFI memory map
61                                     pointed to in previous entry.
62
63linux,uefi-mmap-desc-size   32-bit   Size in bytes of each entry in the UEFI
64                                     memory map.
65
66linux,uefi-mmap-desc-ver    32-bit   Version of the mmap descriptor format.
67
68kaslr-seed                  64-bit   Entropy used to randomize the kernel image
69                                     base address location.
70
71bootargs                    String   Kernel command line
72==========================  ======   ===========================================
73