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        - restricted-dma-pool: This indicates a region of memory meant to be
55          used as a pool of restricted DMA buffers for a set of devices. The
56          memory region would be the only region accessible to those devices.
57          When using this, the no-map and reusable properties must not be set,
58          so the operating system can create a virtual mapping that will be used
59          for synchronization. The main purpose for restricted DMA is to
60          mitigate the lack of DMA access control on systems without an IOMMU,
61          which could result in the DMA accessing the system memory at
62          unexpected times and/or unexpected addresses, possibly leading to data
63          leakage or corruption. The feature on its own provides a basic level
64          of protection against the DMA overwriting buffer contents at
65          unexpected times. However, to protect against general data leakage and
66          system memory corruption, the system needs to provide way to lock down
67          the memory access, e.g., MPU. Note that since coherent allocation
68          needs remapping, one must set up another device coherent pool by
69          shared-dma-pool and use dma_alloc_from_dev_coherent instead for atomic
70          coherent allocation.
71        - vendor specific string in the form <vendor>,[<device>-]<usage>
72no-map (optional) - empty property
73    - Indicates the operating system must not create a virtual mapping
74      of the region as part of its standard mapping of system memory,
75      nor permit speculative access to it under any circumstances other
76      than under the control of the device driver using the region.
77reusable (optional) - empty property
78    - The operating system can use the memory in this region with the
79      limitation that the device driver(s) owning the region need to be
80      able to reclaim it back. Typically that means that the operating
81      system can use that region to store volatile or cached data that
82      can be otherwise regenerated or migrated elsewhere.
83
84A node must not carry both the no-map and the reusable property as these are
85logically contradictory.
86
87Linux implementation note:
88- If a "linux,cma-default" property is present, then Linux will use the
89  region for the default pool of the contiguous memory allocator.
90
91- If a "linux,dma-default" property is present, then Linux will use the
92  region for the default pool of the consistent DMA allocator.
93
94Device node references to reserved memory
95-----------------------------------------
96Regions in the /reserved-memory node may be referenced by other device
97nodes by adding a memory-region property to the device node.
98
99memory-region (optional) - phandle, specifier pairs to children of /reserved-memory
100memory-region-names (optional) - a list of names, one for each corresponding
101  entry in the memory-region property
102
103Example
104-------
105This example defines 4 contiguous regions for Linux kernel:
106one default of all device drivers (named linux,cma@72000000 and 64MiB in size),
107one dedicated to the framebuffer device (named framebuffer@78000000, 8MiB),
108one for multimedia processing (named multimedia-memory@77000000, 64MiB), and
109one for restricted dma pool (named restricted_dma_reserved@0x50000000, 64MiB).
110
111/ {
112	#address-cells = <1>;
113	#size-cells = <1>;
114
115	memory {
116		reg = <0x40000000 0x40000000>;
117	};
118
119	reserved-memory {
120		#address-cells = <1>;
121		#size-cells = <1>;
122		ranges;
123
124		/* global autoconfigured region for contiguous allocations */
125		linux,cma {
126			compatible = "shared-dma-pool";
127			reusable;
128			size = <0x4000000>;
129			alignment = <0x2000>;
130			linux,cma-default;
131		};
132
133		display_reserved: framebuffer@78000000 {
134			reg = <0x78000000 0x800000>;
135		};
136
137		multimedia_reserved: multimedia@77000000 {
138			compatible = "acme,multimedia-memory";
139			reg = <0x77000000 0x4000000>;
140		};
141
142		restricted_dma_reserved: restricted_dma_reserved {
143			compatible = "restricted-dma-pool";
144			reg = <0x50000000 0x4000000>;
145		};
146	};
147
148	/* ... */
149
150	fb0: video@12300000 {
151		memory-region = <&display_reserved>;
152		/* ... */
153	};
154
155	scaler: scaler@12500000 {
156		memory-region = <&multimedia_reserved>;
157		/* ... */
158	};
159
160	codec: codec@12600000 {
161		memory-region = <&multimedia_reserved>;
162		/* ... */
163	};
164
165	pcie_device: pcie_device@0,0 {
166		reg = <0x83010000 0x0 0x00000000 0x0 0x00100000
167		       0x83010000 0x0 0x00100000 0x0 0x00100000>;
168		memory-region = <&restricted_dma_reserved>;
169		/* ... */
170	};
171};
172