On 03/08/2017 10:19 PM, Russell King - ARM Linux wrote: > On Wed, Mar 08, 2017 at 09:44:17PM +0100, Lars-Peter Clausen wrote: >> On 03/08/2017 08:59 PM, Russell King - ARM Linux wrote: >>> On Wed, Mar 08, 2017 at 08:48:31PM +0100, Lars-Peter Clausen wrote: >>>> When the DMA memory is mapped for reading from the device the associated >>>> cachelines are invalidated without writeback. There is no guarantee that >>>> the changes made to the devres_node have made it to main memory yet, or >>>> is there? >>> >>> That is incorrect. >>> >>> Overlapping cache lines are always written back on transitions from CPU >>> to device ownership of the buffer (eg, dma_map_*().) >> >> On ARM. But my understanding is that this is not a universal requirement >> according to DMA-API.txt. It says that mappings must be cache line aligned >> and otherwise behavior is undefined. > > There is no use of the term "undefined" in the document you refer to. > > There is the recommendation that regions are cache line aligned, but > there is quite a bit of history in the kernel where DMA has been to > regions that are not cache line aligned, and where the DMA region > overlaps with data that has recent accesses made to it. I says: "Warnings: Memory coherency operates at a granularity called the cache line width. In order for memory mapped by this API to operate correctly, the mapped region must begin exactly on a cache line boundary and end exactly on one." That doesn't sound like a recommendation to me. "should" usually implies a recommendation while "must" indicates a hard requirement. I believe e.g. MIPS will align the address by masking the lower bits off, without any flushes. Wouldn't be surprised if other architectures do the same. > > The situation is improving (in that DMA buffers are being allocated > separately, rather than being part of some other structure) but that > doesn't mean that it's safe to assume that overlapping cache lines can > be invalidated. > > In any case, DMA with devm allocated buffers really is not a good idea. I very much agree with that part. -- To unsubscribe from this list: send the line "unsubscribe dmaengine" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
