linux-imx

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Dynamic DMA mapping Guide

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:Author: David S. Miller <davem@redhat.com>

:Author: Richard Henderson <rth@cygnus.com>

:Author: Jakub Jelinek <jakub@redhat.com>

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This is a guide to device driver writers on how to use the DMA API

with example pseudo-code.  For a concise description of the API, see

DMA-API.txt.

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CPU and DMA addresses

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There are several kinds of addresses involved in the DMA API, and it's

important to understand the differences.

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The kernel normally uses virtual addresses.  Any address returned by

kmalloc(), vmalloc(), and similar interfaces is a virtual address and can

be stored in a ``void *``.

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The virtual memory system (TLB, page tables, etc.) translates virtual

addresses to CPU physical addresses, which are stored as "phys_addr_t" or

"resource_size_t".  The kernel manages device resources like registers as

physical addresses.  These are the addresses in /proc/iomem.  The physical

address is not directly useful to a driver; it must use ioremap() to map

the space and produce a virtual address.

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I/O devices use a third kind of address: a "bus address".  If a device has

registers at an MMIO address, or if it performs DMA to read or write system

memory, the addresses used by the device are bus addresses.  In some

systems, bus addresses are identical to CPU physical addresses, but in

general they are not.  IOMMUs and host bridges can produce arbitrary

mappings between physical and bus addresses.

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From a device's point of view, DMA uses the bus address space, but it may

be restricted to a subset of that space.  For example, even if a system

supports 64-bit addresses for main memory and PCI BARs, it may use an IOMMU

so devices only need to use 32-bit DMA addresses.

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Here's a picture and some examples::

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               CPU                  CPU                  Bus

             Virtual              Physical             Address

             Address              Address               Space

              Space                Space

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            +-------+             +------+             +------+

            |       |             |MMIO  |   Offset    |      |

            |       |  Virtual    |Space |   applied   |      |

          C +-------+ --------> B +------+ ----------> +------+ A

            |       |  mapping    |      |   by host   |      |

  +-----+   |       |             |      |   bridge    |      |   +--------+

  |     |   |       |             +------+             |      |   |        |

  | CPU |   |       |             | RAM  |             |      |   | Device |

  |     |   |       |             |      |             |      |   |        |

  +-----+   +-------+             +------+             +------+   +--------+

            |       |  Virtual    |Buffer|   Mapping   |      |

          X +-------+ --------> Y +------+ <---------- +------+ Z

            |       |  mapping    | RAM  |   by IOMMU

            |       |             |      |

            |       |             |      |