Linux Device Drivers, Second Edition

Direct Memory Access and Bus Mastering

Direct memory access, or DMA, is the advanced topic that completes our overview of memory issues. DMA is the hardware mechanism that allows peripheral components to transfer their I/O data directly to and from main memory without the need for the system processor to be involved in the transfer. Use of this mechanism can greatly increase throughput to and from a device, because a great deal of computational overhead is eliminated.

To exploit the DMA capabilities of its hardware, the device driver needs to be able to correctly set up the DMA transfer and synchronize with the hardware. Unfortunately, because of its hardware nature, DMA is very system dependent. Each architecture has its own techniques to manage DMA transfers, and the programming interface is different for each. The kernel can’t offer a unified interface, either, because a driver can’t abstract too much from the underlying hardware mechanisms. Some steps have been made in that direction, however, in recent kernels.

This chapter concentrates mainly on the PCI bus, since it is currently the most popular peripheral bus available. Many of the concepts are more widely applicable, though. We also touch on how some other buses, such as ISA and SBus, handle DMA.

Overview of a DMA Data Transfer

Before introducing the programming details, let’s review how a DMA transfer takes place, considering only input transfers to simplify the discussion.

Data transfer can be triggered in two ...