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doc: arm: add UEFI support documentation
This patch provides documentation of the [U]EFI runtime service and
configuration features for the arm architecture.
Cc: Rob Landley <rob@landley.net>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
Acked-by: Grant Likely <grant.likely@linaro.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>
authored by
Leif Lindholm
and committed by
Matt Fleming
e1977464
f84d0275
+66
+2
Documentation/arm/00-INDEX
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Documentation/arm/00-INDEX
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Documentation/arm/uefi.txt
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Documentation/arm/uefi.txt
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UEFI, the Unified Extensible Firmware Interface, is a specification
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governing the behaviours of compatible firmware interfaces. It is
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maintained by the UEFI Forum - http://www.uefi.org/.
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UEFI is an evolution of its predecessor 'EFI', so the terms EFI and
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UEFI are used somewhat interchangeably in this document and associated
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source code. As a rule, anything new uses 'UEFI', whereas 'EFI' refers
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to legacy code or specifications.
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UEFI support in Linux
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=====================
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Booting on a platform with firmware compliant with the UEFI specification
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makes it possible for the kernel to support additional features:
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- UEFI Runtime Services
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- Retrieving various configuration information through the standardised
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interface of UEFI configuration tables. (ACPI, SMBIOS, ...)
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For actually enabling [U]EFI support, enable:
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- CONFIG_EFI=y
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- CONFIG_EFI_VARS=y or m
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The implementation depends on receiving information about the UEFI environment
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in a Flattened Device Tree (FDT) - so is only available with CONFIG_OF.
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UEFI stub
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=========
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The "stub" is a feature that extends the Image/zImage into a valid UEFI
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PE/COFF executable, including a loader application that makes it possible to
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load the kernel directly from the UEFI shell, boot menu, or one of the
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lightweight bootloaders like Gummiboot or rEFInd.
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The kernel image built with stub support remains a valid kernel image for
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booting in non-UEFI environments.
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UEFI kernel support on ARM
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==========================
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UEFI kernel support on the ARM architectures (arm and arm64) is only available
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when boot is performed through the stub.
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When booting in UEFI mode, the stub deletes any memory nodes from a provided DT.
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Instead, the kernel reads the UEFI memory map.
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The stub populates the FDT /chosen node with (and the kernel scans for) the
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following parameters:
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________________________________________________________________________________
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Name | Size | Description
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================================================================================
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linux,uefi-system-table | 64-bit | Physical address of the UEFI System Table.
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--------------------------------------------------------------------------------
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linux,uefi-mmap-start | 64-bit | Physical address of the UEFI memory map,
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| | populated by the UEFI GetMemoryMap() call.
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--------------------------------------------------------------------------------
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linux,uefi-mmap-size | 32-bit | Size in bytes of the UEFI memory map
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| | pointed to in previous entry.
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--------------------------------------------------------------------------------
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linux,uefi-mmap-desc-size | 32-bit | Size in bytes of each entry in the UEFI
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| | memory map.
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--------------------------------------------------------------------------------
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linux,uefi-mmap-desc-ver | 32-bit | Version of the mmap descriptor format.
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--------------------------------------------------------------------------------
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linux,uefi-stub-kern-ver | string | Copy of linux_banner from build.
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--------------------------------------------------------------------------------
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For verbose debug messages, specify 'uefi_debug' on the kernel command line.