172 lines
6.6 KiB
Plaintext
172 lines
6.6 KiB
Plaintext
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*** Reserved memory regions ***
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Reserved memory is specified as a node under the /reserved-memory node.
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The operating system shall exclude reserved memory from normal usage
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one can create child nodes describing particular reserved (excluded from
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normal use) memory regions. Such memory regions are usually designed for
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the special usage by various device drivers.
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Parameters for each memory region can be encoded into the device tree
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with the following nodes:
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/reserved-memory node
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---------------------
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#address-cells, #size-cells (required) - standard definition
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- Should use the same values as the root node
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ranges (required) - standard definition
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- Should be empty
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/reserved-memory/ child nodes
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-----------------------------
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Each child of the reserved-memory node specifies one or more regions of
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reserved memory. Each child node may either use a 'reg' property to
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specify a specific range of reserved memory, or a 'size' property with
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optional constraints to request a dynamically allocated block of memory.
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Following the generic-names recommended practice, node names should
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reflect the purpose of the node (ie. "framebuffer" or "dma-pool"). Unit
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address (@<address>) should be appended to the name if the node is a
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static allocation.
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Properties:
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Requires either a) or b) below.
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a) static allocation
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reg (required) - standard definition
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b) dynamic allocation
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size (required) - length based on parent's #size-cells
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- Size in bytes of memory to reserve.
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alignment (optional) - length based on parent's #size-cells
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- Address boundary for alignment of allocation.
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alloc-ranges (optional) - prop-encoded-array (address, length pairs).
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- Specifies regions of memory that are
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acceptable to allocate from.
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If both reg and size are present, then the reg property takes precedence
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and size is ignored.
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Additional properties:
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compatible (optional) - standard definition
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- may contain the following strings:
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- shared-dma-pool: This indicates a region of memory meant to be
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used as a shared pool of DMA buffers for a set of devices. It can
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be used by an operating system to instantiate the necessary pool
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management subsystem if necessary.
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- restricted-dma-pool: This indicates a region of memory meant to be
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used as a pool of restricted DMA buffers for a set of devices. The
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memory region would be the only region accessible to those devices.
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When using this, the no-map and reusable properties must not be set,
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so the operating system can create a virtual mapping that will be used
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for synchronization. The main purpose for restricted DMA is to
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mitigate the lack of DMA access control on systems without an IOMMU,
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which could result in the DMA accessing the system memory at
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unexpected times and/or unexpected addresses, possibly leading to data
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leakage or corruption. The feature on its own provides a basic level
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of protection against the DMA overwriting buffer contents at
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unexpected times. However, to protect against general data leakage and
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system memory corruption, the system needs to provide way to lock down
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the memory access, e.g., MPU. Note that since coherent allocation
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needs remapping, one must set up another device coherent pool by
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shared-dma-pool and use dma_alloc_from_dev_coherent instead for atomic
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coherent allocation.
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- vendor specific string in the form <vendor>,[<device>-]<usage>
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no-map (optional) - empty property
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- Indicates the operating system must not create a virtual mapping
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of the region as part of its standard mapping of system memory,
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nor permit speculative access to it under any circumstances other
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than under the control of the device driver using the region.
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reusable (optional) - empty property
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- The operating system can use the memory in this region with the
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limitation that the device driver(s) owning the region need to be
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able to reclaim it back. Typically that means that the operating
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system can use that region to store volatile or cached data that
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can be otherwise regenerated or migrated elsewhere.
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A node must not carry both the no-map and the reusable property as these are
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logically contradictory.
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Linux implementation note:
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- If a "linux,cma-default" property is present, then Linux will use the
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region for the default pool of the contiguous memory allocator.
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- If a "linux,dma-default" property is present, then Linux will use the
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region for the default pool of the consistent DMA allocator.
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Device node references to reserved memory
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-----------------------------------------
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Regions in the /reserved-memory node may be referenced by other device
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nodes by adding a memory-region property to the device node.
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memory-region (optional) - phandle, specifier pairs to children of /reserved-memory
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memory-region-names (optional) - a list of names, one for each corresponding
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entry in the memory-region property
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Example
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-------
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This example defines 4 contiguous regions for Linux kernel:
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one default of all device drivers (named linux,cma@72000000 and 64MiB in size),
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one dedicated to the framebuffer device (named framebuffer@78000000, 8MiB),
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one for multimedia processing (named multimedia-memory@77000000, 64MiB), and
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one for restricted dma pool (named restricted_dma_reserved@0x50000000, 64MiB).
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/ {
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#address-cells = <1>;
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#size-cells = <1>;
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memory {
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reg = <0x40000000 0x40000000>;
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};
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reserved-memory {
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#address-cells = <1>;
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#size-cells = <1>;
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ranges;
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/* global autoconfigured region for contiguous allocations */
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linux,cma {
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compatible = "shared-dma-pool";
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reusable;
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size = <0x4000000>;
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alignment = <0x2000>;
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linux,cma-default;
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};
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display_reserved: framebuffer@78000000 {
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reg = <0x78000000 0x800000>;
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};
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multimedia_reserved: multimedia@77000000 {
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compatible = "acme,multimedia-memory";
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reg = <0x77000000 0x4000000>;
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};
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restricted_dma_reserved: restricted_dma_reserved {
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compatible = "restricted-dma-pool";
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reg = <0x50000000 0x4000000>;
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};
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};
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/* ... */
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fb0: video@12300000 {
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memory-region = <&display_reserved>;
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/* ... */
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};
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scaler: scaler@12500000 {
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memory-region = <&multimedia_reserved>;
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/* ... */
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};
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codec: codec@12600000 {
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memory-region = <&multimedia_reserved>;
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/* ... */
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};
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pcie_device: pcie_device@0,0 {
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reg = <0x83010000 0x0 0x00000000 0x0 0x00100000
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0x83010000 0x0 0x00100000 0x0 0x00100000>;
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memory-region = <&restricted_dma_reserved>;
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/* ... */
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};
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};
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