The 2.6 kernel is a stable series which, in theory, should be dedicated to
the fixing of bugs rather than changing APIs. Anybody who risks thinking
that things have become too stable, however, need only look at
this massive patch from David Miller, which
changes the DMA API and touches a full 100 files. This patch had done a
little time in the -mm tree, but had never really been discussed on the
mailing lists before its inclusion.
The change is in the "synchronization" calls that the DMA layer provides
for streaming mappings. A streaming mapping is a short-lived structure set
up to support one or more direct memory access operations; depending on the
architecture, setting up a streaming mapping can involve creating bounce
buffers, programming I/O memory management unit (IOMMU) registers, flushing
processor caches, and more. These mappings have strict rules about the
"ownership" of the buffer; when a streaming mapping is created, it is owned
by the device, and the processor cannot touch it. If a device driver
ignores that rule, it risks corrupting data in a number of ways.
It is sometimes necessary, however, to allow the processor to access a mapped
streaming DMA buffer. To that end, the DMA layer has long provided a
set of functions (like dma_sync_single() and
pci_sync_single()) which transfer ownership of the buffer to
the CPU. What has always been lacking, however, is a way to transfer
ownership back to the device. To fill in that gap, the various
synchronization functions have been split in two; instead of
dma_sync_single() a driver must now call one or both of:
dma_sync_single_for_cpu(struct device *dev,
dma_addr_t dma_handle,
size_t size,
enum dma_data_direction direction);
dma_sync_single_for_device(struct device *dev,
dma_addr_t dma_handle,
size_t size,
enum dma_data_direction direction);
dma_sync_single_for_cpu() gives ownership of the DMA buffer back
to the processor. After that call, driver code can read or modify the
buffer, but the device should not touch it. A call to
dma_sync_single_for_device() is required to allow the device to
access the buffer again. The other synchronization functions (for
scatter/gather and DAC mappings) have been changed as well.
As might be expected from a change like this, the result was a lot of
broken drivers. The patch fixes the in-tree users of the discontinued DMA
functions. Out-of-tree and binary-only drivers, however, will have to be
fixed separately.
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