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diff --git a/Documentation/arm/uefi.txt b/Documentation/arm/uefi.txt new file mode 100644 index 000000000000..d60030a1b909 --- /dev/null +++ b/Documentation/arm/uefi.txt @@ -0,0 +1,64 @@ +UEFI, the Unified Extensible Firmware Interface, is a specification +governing the behaviours of compatible firmware interfaces. It is +maintained by the UEFI Forum - http://www.uefi.org/. + +UEFI is an evolution of its predecessor 'EFI', so the terms EFI and +UEFI are used somewhat interchangeably in this document and associated +source code. As a rule, anything new uses 'UEFI', whereas 'EFI' refers +to legacy code or specifications. + +UEFI support in Linux +===================== +Booting on a platform with firmware compliant with the UEFI specification +makes it possible for the kernel to support additional features: +- UEFI Runtime Services +- Retrieving various configuration information through the standardised + interface of UEFI configuration tables. (ACPI, SMBIOS, ...) + +For actually enabling [U]EFI support, enable: +- CONFIG_EFI=y +- CONFIG_EFI_VARS=y or m + +The implementation depends on receiving information about the UEFI environment +in a Flattened Device Tree (FDT) - so is only available with CONFIG_OF. + +UEFI stub +========= +The "stub" is a feature that extends the Image/zImage into a valid UEFI +PE/COFF executable, including a loader application that makes it possible to +load the kernel directly from the UEFI shell, boot menu, or one of the +lightweight bootloaders like Gummiboot or rEFInd. + +The kernel image built with stub support remains a valid kernel image for +booting in non-UEFI environments. + +UEFI kernel support on ARM +========================== +UEFI kernel support on the ARM architectures (arm and arm64) is only available +when boot is performed through the stub. + +When booting in UEFI mode, the stub deletes any memory nodes from a provided DT. +Instead, the kernel reads the UEFI memory map. + +The stub populates the FDT /chosen node with (and the kernel scans for) the +following parameters: +________________________________________________________________________________ +Name | Size | Description +================================================================================ +linux,uefi-system-table | 64-bit | Physical address of the UEFI System Table. +-------------------------------------------------------------------------------- +linux,uefi-mmap-start | 64-bit | Physical address of the UEFI memory map, + | | populated by the UEFI GetMemoryMap() call. +-------------------------------------------------------------------------------- +linux,uefi-mmap-size | 32-bit | Size in bytes of the UEFI memory map + | | pointed to in previous entry. +-------------------------------------------------------------------------------- +linux,uefi-mmap-desc-size | 32-bit | Size in bytes of each entry in the UEFI + | | memory map. +-------------------------------------------------------------------------------- +linux,uefi-mmap-desc-ver | 32-bit | Version of the mmap descriptor format. +-------------------------------------------------------------------------------- +linux,uefi-stub-kern-ver | string | Copy of linux_banner from build. +-------------------------------------------------------------------------------- + +For verbose debug messages, specify 'uefi_debug' on the kernel command line. |