linux/Documentation/arm/uefi.rst
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   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                        Size     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
  68linux,initrd-start          64-bit   Physical start address of an initrd
  69
  70linux,initrd-end            64-bit   Physical end address of an initrd
  71
  72kaslr-seed                  64-bit   Entropy used to randomize the kernel image
  73                                     base address location.
  74==========================  ======   ===========================================
  75