Specifications

This is the Title of the Book, eMatter Edition

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Chapter 15: Memory Mapping and DMA

coherency in the hardware, but others require software support. The generic DMA

layer goes to great lengths to ensure that things work correctly on all architectures,

but, as we will see, proper behavior requires adherence to a small set of rules.

The DMA mapping sets up a new type,

dma_addr_t, to represent bus addresses. Vari-

ables of type

dma_addr_t should be treated as opaque by the driver; the only allow-

able operations are to pass them to the DMA support routines and to the device

itself. As a bus address,

dma_addr_t may lead to unexpected problems if used directly

by the CPU.

The PCI code distinguishes between two types of DMA mappings, depending on

how long the DMA buffer is expected to stay around:

Coherent DMA mappings

These mappings usually exist for the life of the driver. A coherent buffer must be

simultaneously available to both the CPU and the peripheral (other types of

mappings, as we will see later, can be available only to one or the other at any

given time). As a result, coherent mappings must live in cache-coherent mem-

ory. Coherent mappings can be expensive to set up and use.

Streaming DMA mappings

Streaming mappings are usually set up for a single operation. Some architec-

tures allow for significant optimizations when streaming mappings are used, as

we see, but these mappings also are subject to a stricter set of rules in how they

may be accessed. The kernel developers recommend the use of streaming map-

pings over coherent mappings whenever possible. There are two reasons for this

recommendation. The first is that, on systems that support mapping registers,

each DMA mapping uses one or more of them on the bus. Coherent mappings,

which have a long lifetime, can monopolize these registers for a long time, even

when they are not being used. The other reason is that, on some hardware,

streaming mappings can be optimized in ways that are not available to coherent

mappings.

The two mapping types must be manipulated in different ways; it’s time to look at

the details.

Setting up coherent DMA mappings

A driver can set up a coherent mapping with a call to dma_alloc_coherent:

void *dma_alloc_coherent(struct device *dev, size_t size,

dma_addr_t *dma_handle, int flag);

This function handles both the allocation and the mapping of the buffer. The first two

arguments are the device structure and the size of the buffer needed. The function

returns the result of the DMA mapping in two places. The return value from the func-

tion is a kernel virtual address for the buffer, which may be used by the driver; the

associated bus address, meanwhile, is returned in

dma_handle. Allocation is handled in

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