Kernel Korner - Using DMA

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:

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.

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.