b4ba5e5eaf
The bootargs node is also added by the EFI stub in the function update_fdt(), so add it to the table. Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com> Reviewed-by: Atish Patra <atishp@rivosinc.com> Reviewed-by: Song Shuai <songshuaishuai@tinylab.org> Signed-off-by: Jonathan Corbet <corbet@lwn.net> Link: https://lore.kernel.org/r/20230817130734.10387-1-alexghiti@rivosinc.com
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================================================
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The Unified Extensible Firmware Interface (UEFI)
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================================================
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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_EFIVAR_FS=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 Type 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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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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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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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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linux,uefi-mmap-desc-ver 32-bit Version of the mmap descriptor format.
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kaslr-seed 64-bit Entropy used to randomize the kernel image
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base address location.
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bootargs String Kernel command line
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========================== ====== ===========================================
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