xref: /kernel/linux/linux-5.10/Documentation/ABI/testing/sysfs-firmware-memmap

What:		/sys/firmware/memmap/
Date:		June 2008
Contact:	Bernhard Walle <bernhard.walle@gmx.de>
Description:
		On all platforms, the firmware provides a memory map which the
		kernel reads. The resources from that memory map are registered
		in the kernel resource tree and exposed to userspace via
		/proc/iomem (together with other resources).

		However, on most architectures that firmware-provided memory
		map is modified afterwards by the kernel itself, either because
		the kernel merges that memory map with other information or
		just because the user overwrites that memory map via command
		line.

		kexec needs the raw firmware-provided memory map to setup the
		parameter segment of the kernel that should be booted with
		kexec. Also, the raw memory map is useful for debugging. For
		that reason, /sys/firmware/memmap is an interface that provides
		the raw memory map to userspace.

		The structure is as follows: Under /sys/firmware/memmap there
		are subdirectories with the number of the entry as their name::

			/sys/firmware/memmap/0
			/sys/firmware/memmap/1
			/sys/firmware/memmap/2
			/sys/firmware/memmap/3
			...

		The maximum depends on the number of memory map entries provided
		by the firmware. The order is just the order that the firmware
		provides.

		Each directory contains three files:

		========  =====================================================
		start	  The start address (as hexadecimal number with the
			  '0x' prefix).
		end	  The end address, inclusive (regardless whether the
			  firmware provides inclusive or exclusive ranges).
		type	  Type of the entry as string. See below for a list of
			  valid types.
		========  =====================================================

		So, for example::

			/sys/firmware/memmap/0/start
			/sys/firmware/memmap/0/end
			/sys/firmware/memmap/0/type
			/sys/firmware/memmap/1/start
			...

		Currently following types exist:

		  - System RAM
		  - ACPI Tables
		  - ACPI Non-volatile Storage
		  - reserved

		Following shell snippet can be used to display that memory
		map in a human-readable format::

		  #!/bin/bash
		  cd /sys/firmware/memmap
		  for dir in * ; do
		      start=$(cat $dir/start)
		      end=$(cat $dir/end)
		      type=$(cat $dir/type)
		      printf "%016x-%016x (%s)\n" $start $[ $end +1] "$type"
		  done