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