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Diffstat (limited to 'Documentation/memory-barriers.txt')
| -rw-r--r-- | Documentation/memory-barriers.txt | 42 |
1 files changed, 42 insertions, 0 deletions
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index 7ee2ae6d5451..70a09f8a0383 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt @@ -1633,6 +1633,48 @@ There are some more advanced barrier functions: operations" subsection for information on where to use these. + (*) dma_wmb(); + (*) dma_rmb(); + + 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. + + For example, consider a device driver that shares memory with a device + and uses a descriptor status value to indicate if the descriptor belongs + to the device or the CPU, and a doorbell to notify it when new + descriptors are available: + + if (desc->status != DEVICE_OWN) { + /* do not read data until we own descriptor */ + dma_rmb(); + + /* read/modify data */ + read_data = desc->data; + desc->data = write_data; + + /* flush modifications before status update */ + dma_wmb(); + + /* assign ownership */ + desc->status = DEVICE_OWN; + + /* force memory to sync before notifying device via MMIO */ + wmb(); + + /* notify device of new descriptors */ + writel(DESC_NOTIFY, doorbell); + } + + The dma_rmb() allows us guarantee the device has released ownership + before we read the data from the descriptor, and he dma_wmb() allows + us to guarantee the data is written to the descriptor before the device + can see it now has ownership. The wmb() is needed to guarantee that the + cache coherent memory writes have completed before attempting a write to + the cache incoherent MMIO region. + + See Documentation/DMA-API.txt for more information on consistent memory. + MMIO WRITE BARRIER ------------------ |