Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt

   1 *** Reserved memory regions ***
   2
   3 Reserved memory is specified as a node under the /reserved-memory node.
   4 The operating system shall exclude reserved memory from normal usage
   5 one can create child nodes describing particular reserved (excluded from
   6 normal use) memory regions. Such memory regions are usually designed for
   7 the special usage by various device drivers.
   8
   9 Parameters for each memory region can be encoded into the device tree
  10 with 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
  16 ranges (required) - standard definition
  17     - Should be empty
  18
  19 /reserved-memory/ child nodes
  20 -----------------------------
  21 Each child of the reserved-memory node specifies one or more regions of
  22 reserved memory. Each child node may either use a 'reg' property to
  23 specify a specific range of reserved memory, or a 'size' property with
  24 optional constraints to request a dynamically allocated block of memory.
  25
  26 Following the generic-names recommended practice, node names should
  27 reflect the purpose of the node (ie. "framebuffer" or "dma-pool"). Unit
  28 address (@<address>) should be appended to the name if the node is a
  29 static allocation.
  30
  31 Properties:
  32 Requires either a) or b) below.
  33 a) static allocation
  34    reg (required) - standard definition
  35 b) 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
  44 If both reg and size are present, then the reg property takes precedence
  45 and size is ignored.
  46
  47 Additional properties:
  48 compatible (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 instanciate the necessary pool
  53           management subsystem if necessary.
  54         - vendor specific string in the form <vendor>,[<device>-]<usage>
  55 no-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.
  60 reusable (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
  67 Linux implementation note:
  68 - If a "linux,cma-default" property is present, then Linux will use the
  69   region for the default pool of the contiguous memory allocator.
  70
  71 Device node references to reserved memory
  72 -----------------------------------------
  73 Regions in the /reserved-memory node may be referenced by other device
  74 nodes by adding a memory-region property to the device node.
  75
  76 memory-region (optional) - phandle, specifier pairs to children of /reserved-memory
  77
  78 Example
  79 -------
  80 This example defines 3 contiguous regions are defined for Linux kernel:
  81 one default of all device drivers (named linux,cma@72000000 and 64MiB in size),
  82 one dedicated to the framebuffer device (named framebuffer@78000000, 8MiB), and
  83 one for multimedia processing (named multimedia-memory@77000000, 64MiB).
  84
  85 / {
  86         #address-cells = <1>;
  87         #size-cells = <1>;
  88
  89         memory {
  90                 reg = <0x40000000 0x40000000>;
  91         };
  92
  93         reserved-memory {
  94                 #address-cells = <1>;
  95                 #size-cells = <1>;
  96                 ranges;
  97
  98                 /* global autoconfigured region for contiguous allocations */
  99                 linux,cma {
 100                         compatible = "shared-dma-pool";
 101                         reusable;
 102                         size = <0x4000000>;
 103                         alignment = <0x2000>;
 104                         linux,cma-default;
 105                 };
 106
 107                 display_reserved: framebuffer@78000000 {
 108                         reg = <0x78000000 0x800000>;
 109                 };
 110
 111                 multimedia_reserved: multimedia@77000000 {
 112                         compatible = "acme,multimedia-memory";
 113                         reg = <0x77000000 0x4000000>;
 114                 };
 115         };
 116
 117         /* ... */
 118
 119         fb0: video@12300000 {
 120                 memory-region = <&display_reserved>;
 121                 /* ... */
 122         };
 123
 124         scaler: scaler@12500000 {
 125                 memory-region = <&multimedia_reserved>;
 126                 /* ... */
 127         };
 128
 129         codec: codec@12600000 {
 130                 memory-region = <&multimedia_reserved>;
 131                 /* ... */
 132         };
 133 };