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1================================================2The Unified Extensible Firmware Interface (UEFI)3================================================4 5UEFI, the Unified Extensible Firmware Interface, is a specification6governing the behaviours of compatible firmware interfaces. It is7maintained by the UEFI Forum - http://www.uefi.org/.8 9UEFI is an evolution of its predecessor 'EFI', so the terms EFI and10UEFI are used somewhat interchangeably in this document and associated11source code. As a rule, anything new uses 'UEFI', whereas 'EFI' refers12to legacy code or specifications.13 14UEFI support in Linux15=====================16Booting on a platform with firmware compliant with the UEFI specification17makes it possible for the kernel to support additional features:18 19- UEFI Runtime Services20- Retrieving various configuration information through the standardised21 interface of UEFI configuration tables. (ACPI, SMBIOS, ...)22 23For actually enabling [U]EFI support, enable:24 25- CONFIG_EFI=y26- CONFIG_EFIVAR_FS=y or m27 28The implementation depends on receiving information about the UEFI environment29in a Flattened Device Tree (FDT) - so is only available with CONFIG_OF.30 31UEFI stub32=========33The "stub" is a feature that extends the Image/zImage into a valid UEFI34PE/COFF executable, including a loader application that makes it possible to35load the kernel directly from the UEFI shell, boot menu, or one of the36lightweight bootloaders like Gummiboot or rEFInd.37 38The kernel image built with stub support remains a valid kernel image for39booting in non-UEFI environments.40 41UEFI kernel support on ARM42==========================43UEFI kernel support on the ARM architectures (arm and arm64) is only available44when 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) the50following parameters:51 52========================== ====== ===========================================53Name Type Description54========================== ====== ===========================================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 map61 pointed to in previous entry.62 63linux,uefi-mmap-desc-size 32-bit Size in bytes of each entry in the UEFI64 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 image69 base address location.70 71bootargs String Kernel command line72========================== ====== ===========================================73