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