Kernel Korner - Using DMA

DMA makes I/O go faster by letting devices read and write memory without bothering the CPU. Here's how the kernel keeps track of changes that happen behind the CPU's back.

where the arguments are identical to dma_unmap_sg.

The most important factor in accessing data is when you do it. The rules for accessing depend on dir:

  • DMA_TO_DEVICE: the API must be called after modifying the data but before sending it to the device.

  • DMA_FROM_DEVICE: the API must be called after the device has returned the data but before the driver attempts to read it.

  • DMA_BIDIRECTIONAL: the API may need to be called twice, after modifying the data but before sending it to the device and after the device finishes with it but before the driver accesses it again.

Allocating Coherent Memory

Most devices use mailbox-type regions of memory for communication between the device and the driver. The usual characteristic of this mailbox region is that it is never used beyond the device driver. Managing the coherency of the mailbox using the previous API would be quite a chore, so the kernel provides a method for allocating a region of memory guaranteed to be coherent at all times between the device and the CPU:

void
*dma_alloc_coherent(struct device *dev, size_tsize,
                    dma_addr_t *physaddr, int flag);

This returns the virtual address of a coherent region of size that also has a bus physical address (physaddr) to the device. The flag is used to specify the allocation type GFP_KERNEL to indicate the allocation may sleep to obtain the memory and GFP_ATOMIC to indicate the allocation may not sleep and may return NULL if it cannot obtain the memory. All memory allocated by this API also is guaranteed to be contiguous both in virtual and bus physical memory. There is an absolute requirement that size be less than 128KB.

As part of driver removal, the coherent region of memory must be freed with:

void
dma_free_coherent(struct device *dev, size_tsize,
                  void *virtaddr,
                  dma_addr_t *physaddr);

where size is the size of the coherent region and virtaddr and physaddr are the CPU virtual and bus physical addresses, respectively, returned for the coherent region.

Conclusions

The article offers a lightning-quick overview of how the block layer interacts with device drivers to produce SG lists for programming devices. You may find several additional pieces of the DMA API useful, including APIs that handle unfragmented regions of physical memory. If this article whets your appetite, you're now ready to move on to reading the kernel Documents and source.

James Bottomley is the Software Architect for SteelEye. He also is an active member of the Open Source community. He maintains the SCSI subsystem, the Linux Voyager port and the 53c700 driver and has made contributions to PA-RISC Linux development in the area of DMA/device model abstraction.

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