1*** Reserved memory regions ***
2
3Reserved memory is specified as a node under the /reserved-memory node.
4The operating system shall exclude reserved memory from normal usage
5one can create child nodes describing particular reserved (excluded from
6normal use) memory regions. Such memory regions are usually designed for
7the special usage by various device drivers.
8
9Parameters for each memory region can be encoded into the device tree
10with the following nodes:
11
12/reserved-memory node
13---------------------
14#address-cells, #size-cells (required) - standard definition
15    - Should use the same values as the root node
16ranges (required) - standard definition
17    - Should be empty
18
19/reserved-memory/ child nodes
20-----------------------------
21Each child of the reserved-memory node specifies one or more regions of
22reserved memory. Each child node may either use a 'reg' property to
23specify a specific range of reserved memory, or a 'size' property with
24optional constraints to request a dynamically allocated block of memory.
25
26Following the generic-names recommended practice, node names should
27reflect the purpose of the node (ie. "framebuffer" or "dma-pool"). Unit
28address (@<address>) should be appended to the name if the node is a
29static allocation.
30
31Properties:
32Requires either a) or b) below.
33a) static allocation
34   reg (required) - standard definition
35b) dynamic allocation
36   size (required) - length based on parent's #size-cells
37                   - Size in bytes of memory to reserve.
38   alignment (optional) - length based on parent's #size-cells
39                        - Address boundary for alignment of allocation.
40   alloc-ranges (optional) - prop-encoded-array (address, length pairs).
41                           - Specifies regions of memory that are
42                             acceptable to allocate from.
43
44If both reg and size are present, then the reg property takes precedence
45and size is ignored.
46
47Additional properties:
48compatible (optional) - standard definition
49    - may contain the following strings:
50        - shared-dma-pool: This indicates a region of memory meant to be
51          used as a shared pool of DMA buffers for a set of devices. It can
52          be used by an operating system to instantiate the necessary pool
53          management subsystem if necessary.
54        - vendor specific string in the form <vendor>,[<device>-]<usage>
55no-map (optional) - empty property
56    - Indicates the operating system must not create a virtual mapping
57      of the region as part of its standard mapping of system memory,
58      nor permit speculative access to it under any circumstances other
59      than under the control of the device driver using the region.
60reusable (optional) - empty property
61    - The operating system can use the memory in this region with the
62      limitation that the device driver(s) owning the region need to be
63      able to reclaim it back. Typically that means that the operating
64      system can use that region to store volatile or cached data that
65      can be otherwise regenerated or migrated elsewhere.
66
67A node must not carry both the no-map and the reusable property as these are
68logically contradictory.
69
70Linux implementation note:
71- If a "linux,cma-default" property is present, then Linux will use the
72  region for the default pool of the contiguous memory allocator.
73
74- If a "linux,dma-default" property is present, then Linux will use the
75  region for the default pool of the consistent DMA allocator.
76
77Device node references to reserved memory
78-----------------------------------------
79Regions in the /reserved-memory node may be referenced by other device
80nodes by adding a memory-region property to the device node.
81
82memory-region (optional) - phandle, specifier pairs to children of /reserved-memory
83memory-region-names (optional) - a list of names, one for each corresponding
84  entry in the memory-region property
85
86Example
87-------
88This example defines 3 contiguous regions are defined for Linux kernel:
89one default of all device drivers (named linux,cma@72000000 and 64MiB in size),
90one dedicated to the framebuffer device (named framebuffer@78000000, 8MiB), and
91one for multimedia processing (named multimedia-memory@77000000, 64MiB).
92
93/ {
94	#address-cells = <1>;
95	#size-cells = <1>;
96
97	memory {
98		reg = <0x40000000 0x40000000>;
99	};
100
101	reserved-memory {
102		#address-cells = <1>;
103		#size-cells = <1>;
104		ranges;
105
106		/* global autoconfigured region for contiguous allocations */
107		linux,cma {
108			compatible = "shared-dma-pool";
109			reusable;
110			size = <0x4000000>;
111			alignment = <0x2000>;
112			linux,cma-default;
113		};
114
115		display_reserved: framebuffer@78000000 {
116			reg = <0x78000000 0x800000>;
117		};
118
119		multimedia_reserved: multimedia@77000000 {
120			compatible = "acme,multimedia-memory";
121			reg = <0x77000000 0x4000000>;
122		};
123	};
124
125	/* ... */
126
127	fb0: video@12300000 {
128		memory-region = <&display_reserved>;
129		/* ... */
130	};
131
132	scaler: scaler@12500000 {
133		memory-region = <&multimedia_reserved>;
134		/* ... */
135	};
136
137	codec: codec@12600000 {
138		memory-region = <&multimedia_reserved>;
139		/* ... */
140	};
141};
142