doc/device-tree-bindings/reserved-memory/reserved-memory.txt

*** Reserved memory regions ***

Reserved memory is specified as a node under the /reserved-memory node.
The operating system shall exclude reserved memory from normal usage
one can create child nodes describing particular reserved (excluded from
normal use) memory regions. Such memory regions are usually designed for
the special usage by various device drivers.

Parameters for each memory region can be encoded into the device tree
with the following nodes:

/reserved-memory node
---------------------
#address-cells, #size-cells (required) - standard definition
    - Should use the same values as the root node
ranges (required) - standard definition
    - Should be empty

/reserved-memory/ child nodes
-----------------------------
Each child of the reserved-memory node specifies one or more regions of
reserved memory. Each child node may either use a 'reg' property to
specify a specific range of reserved memory, or a 'size' property with
optional constraints to request a dynamically allocated block of memory.

Following the generic-names recommended practice, node names should
reflect the purpose of the node (ie. "framebuffer" or "dma-pool"). Unit
address (@<address>) should be appended to the name if the node is a
static allocation.

Properties:
Requires either a) or b) below.
a) static allocation
   reg (required) - standard definition
b) dynamic allocation
   size (required) - length based on parent's #size-cells
                   - Size in bytes of memory to reserve.
   alignment (optional) - length based on parent's #size-cells
                        - Address boundary for alignment of allocation.
   alloc-ranges (optional) - prop-encoded-array (address, length pairs).
                           - Specifies regions of memory that are
                             acceptable to allocate from.

If both reg and size are present, then the reg property takes precedence
and size is ignored.

Additional properties:
compatible (optional) - standard definition
    - may contain the following strings:
        - shared-dma-pool: This indicates a region of memory meant to be
          used as a shared pool of DMA buffers for a set of devices. It can
          be used by an operating system to instantiate the necessary pool
          management subsystem if necessary.
        - vendor specific string in the form <vendor>,[<device>-]<usage>
no-map (optional) - empty property
    - Indicates the operating system must not create a virtual mapping
      of the region as part of its standard mapping of system memory,
      nor permit speculative access to it under any circumstances other
      than under the control of the device driver using the region.
reusable (optional) - empty property
    - The operating system can use the memory in this region with the
      limitation that the device driver(s) owning the region need to be
      able to reclaim it back. Typically that means that the operating
      system can use that region to store volatile or cached data that
      can be otherwise regenerated or migrated elsewhere.

Linux implementation note:
- If a "linux,cma-default" property is present, then Linux will use the
  region for the default pool of the contiguous memory allocator.

- If a "linux,dma-default" property is present, then Linux will use the
  region for the default pool of the consistent DMA allocator.

Device node references to reserved memory
-----------------------------------------
Regions in the /reserved-memory node may be referenced by other device
nodes by adding a memory-region property to the device node.

memory-region (optional) - phandle, specifier pairs to children of /reserved-memory

Example
-------
This example defines 3 contiguous regions are defined for Linux kernel:
one default of all device drivers (named linux,cma@72000000 and 64MiB in size),
one dedicated to the framebuffer device (named framebuffer@78000000, 8MiB), and
one for multimedia processing (named multimedia-memory@77000000, 64MiB).

/ {
	#address-cells = <1>;
	#size-cells = <1>;

	memory {
		reg = <0x40000000 0x40000000>;
	};

	reserved-memory {
		#address-cells = <1>;
		#size-cells = <1>;
		ranges;

		/* global autoconfigured region for contiguous allocations */
		linux,cma {
			compatible = "shared-dma-pool";
			reusable;
			size = <0x4000000>;
			alignment = <0x2000>;
			linux,cma-default;
		};

		display_reserved: framebuffer@78000000 {
			reg = <0x78000000 0x800000>;
		};

		multimedia_reserved: multimedia@77000000 {
			compatible = "acme,multimedia-memory";
			reg = <0x77000000 0x4000000>;
		};
	};

	/* ... */

	fb0: video@12300000 {
		memory-region = <&display_reserved>;
		/* ... */
	};

	scaler: scaler@12500000 {
		memory-region = <&multimedia_reserved>;
		/* ... */
	};

	codec: codec@12600000 {
		memory-region = <&multimedia_reserved>;
		/* ... */
	};
};