What is DMA mapping and DMA engine in context of linux kernel?

Question

What is DMA mapping and DMA engine in context of linux kernel? When DMA mapping API and DMA engine API can be used in Linux Device Driver? Any real Linux Device Driver example as a reference would be great.

Answer 1

What is DMA mapping and DMA engine in context of linux kernel?

The kernel normally uses virtual address. Functions like kmalloc(), vmalloc() normally return virtual address. It can be stored in void*. Virtual memory system converts these addresses to physical addresses. These physical addresses are not actually useful to drivers. Drivers must use ioremap() to map the space and produce a virtual address.

               CPU                  CPU                  Bus
             Virtual              Physical             Address
             Address              Address               Space
              Space                Space

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

If device supports DMA , the driver sets up buffer using kmalloc or similar interface which returns virtual address (X). The virtual memory system maps X to a physical address (Y) in system RAM. The driver can use virtual address X to access the buffer, but the device itself cannot because DMA doesn't go through the CPU virtual memory system. In some system only Device can directly do DMA to physical address. In some system IOMMU hardware is used to translate DMA address to physical address.Look at the figure above It translate Z to Y.

When DMA mapping API can be used in Linux Device Driver?

Reason to use DMA mapping API is driver can return virtual address X to interface like dma_map_single(), which sets up any required IOMMU mapping and returns the DMA address Z.The driver then tells the device to do DMA to Z, and the IOMMU maps it to the buffer at address Y in system RAM.

Reference is taken from this link.

Any real Linux Device Driver example as a reference would be great.

A simple PCI DMA example

Inside linux kernel you can look to drivers/dma for various real drivers.