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| -rw-r--r-- | Documentation/memory-barriers.txt | 22 |
1 files changed, 11 insertions, 11 deletions
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index cc621decd943..06e14efd8662 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt @@ -1910,7 +1910,8 @@ There are some more advanced barrier functions: These are for use with consistent memory to guarantee the ordering of writes or reads of shared memory accessible to both the CPU and a - DMA capable device. + DMA capable device. See Documentation/core-api/dma-api.rst file for more + information about consistent memory. For example, consider a device driver that shares memory with a device and uses a descriptor status value to indicate if the descriptor belongs @@ -1931,22 +1932,21 @@ There are some more advanced barrier functions: /* assign ownership */ desc->status = DEVICE_OWN; - /* notify device of new descriptors */ + /* Make descriptor status visible to the device followed by + * notify device of new descriptor + */ writel(DESC_NOTIFY, doorbell); } - The dma_rmb() allows us guarantee the device has released ownership + The dma_rmb() allows us to guarantee that the device has released ownership before we read the data from the descriptor, and the dma_wmb() allows us to guarantee the data is written to the descriptor before the device can see it now has ownership. The dma_mb() implies both a dma_rmb() and - a dma_wmb(). Note that, when using writel(), a prior wmb() is not needed - to guarantee that the cache coherent memory writes have completed before - writing to the MMIO region. The cheaper writel_relaxed() does not provide - this guarantee and must not be used here. - - See the subsection "Kernel I/O barrier effects" for more information on - relaxed I/O accessors and the Documentation/core-api/dma-api.rst file for - more information on consistent memory. + a dma_wmb(). + + Note that the dma_*() barriers do not provide any ordering guarantees for + accesses to MMIO regions. See the later "KERNEL I/O BARRIER EFFECTS" + subsection for more information about I/O accessors and MMIO ordering. (*) pmem_wmb(); |