0001 ================================================
0002 The Unified Extensible Firmware Interface (UEFI)
0003 ================================================
0004
0005 UEFI, the Unified Extensible Firmware Interface, is a specification
0006 governing the behaviours of compatible firmware interfaces. It is
0007 maintained by the UEFI Forum - http://www.uefi.org/.
0008
0009 UEFI is an evolution of its predecessor 'EFI', so the terms EFI and
0010 UEFI are used somewhat interchangeably in this document and associated
0011 source code. As a rule, anything new uses 'UEFI', whereas 'EFI' refers
0012 to legacy code or specifications.
0013
0014 UEFI support in Linux
0015 =====================
0016 Booting on a platform with firmware compliant with the UEFI specification
0017 makes it possible for the kernel to support additional features:
0018
0019 - UEFI Runtime Services
0020 - Retrieving various configuration information through the standardised
0021 interface of UEFI configuration tables. (ACPI, SMBIOS, ...)
0022
0023 For actually enabling [U]EFI support, enable:
0024
0025 - CONFIG_EFI=y
0026 - CONFIG_EFIVAR_FS=y or m
0027
0028 The implementation depends on receiving information about the UEFI environment
0029 in a Flattened Device Tree (FDT) - so is only available with CONFIG_OF.
0030
0031 UEFI stub
0032 =========
0033 The "stub" is a feature that extends the Image/zImage into a valid UEFI
0034 PE/COFF executable, including a loader application that makes it possible to
0035 load the kernel directly from the UEFI shell, boot menu, or one of the
0036 lightweight bootloaders like Gummiboot or rEFInd.
0037
0038 The kernel image built with stub support remains a valid kernel image for
0039 booting in non-UEFI environments.
0040
0041 UEFI kernel support on ARM
0042 ==========================
0043 UEFI kernel support on the ARM architectures (arm and arm64) is only available
0044 when boot is performed through the stub.
0045
0046 When booting in UEFI mode, the stub deletes any memory nodes from a provided DT.
0047 Instead, the kernel reads the UEFI memory map.
0048
0049 The stub populates the FDT /chosen node with (and the kernel scans for) the
0050 following parameters:
0051
0052 ========================== ====== ===========================================
0053 Name Size Description
0054 ========================== ====== ===========================================
0055 linux,uefi-system-table 64-bit Physical address of the UEFI System Table.
0056
0057 linux,uefi-mmap-start 64-bit Physical address of the UEFI memory map,
0058 populated by the UEFI GetMemoryMap() call.
0059
0060 linux,uefi-mmap-size 32-bit Size in bytes of the UEFI memory map
0061 pointed to in previous entry.
0062
0063 linux,uefi-mmap-desc-size 32-bit Size in bytes of each entry in the UEFI
0064 memory map.
0065
0066 linux,uefi-mmap-desc-ver 32-bit Version of the mmap descriptor format.
0067
0068 linux,initrd-start 64-bit Physical start address of an initrd
0069
0070 linux,initrd-end 64-bit Physical end address of an initrd
0071
0072 kaslr-seed 64-bit Entropy used to randomize the kernel image
0073 base address location.
0074 ========================== ====== ===========================================
