On Wed, Jul 14, 2010 at 06:47:34PM -0700, Eric W. Biederman wrote: > Zach Pfeffer <zpfeffer@xxxxxxxxxxxxxx> writes: > > > On Wed, Jul 14, 2010 at 11:05:36PM +0100, Russell King - ARM Linux wrote: > >> On Wed, Jul 14, 2010 at 01:11:49PM -0700, Zach Pfeffer wrote: > >> > If the DMA-API contained functions to allocate virtual space separate > >> > from physical space and reworked how chained buffers functioned it > >> > would probably work - but then things start to look like the VCM API > >> > which does graph based map management. > >> > >> Every additional virtual mapping of a physical buffer results in > >> additional cache aliases on aliasing caches, and more workload for > >> developers to sort out the cache aliasing issues. > >> > >> What does VCM to do mitigate that? > > > > The VCM ensures that all mappings that map a given physical buffer: > > IOMMU mappings, CPU mappings and one-to-one device mappings all map > > that buffer using the same (or compatible) attributes. At this point > > the only attribute that users can pass is CACHED. In the absence of > > CACHED all accesses go straight through to the physical memory. > > > > The architecture of the VCM allows these sorts of consistency checks > > to be made since all mappers of a given physical resource are > > tracked. This is feasible because the physical resources we're > > tracking are typically large. > > On x86 this is implemented in the pat code, and could reasonably be > generalized to be cross platform. > > This is controlled by HAVE_PFNMAP_TRACKING and with entry points > like track_pfn_vma_new. > > Given that we already have an implementation that tracks the cached > vs non-cached attribute using the dma api. I don't see that the > API has to change. An implementation of the cached vs non-cached > status for arm and other architectures is probably appropriate. > > It is definitely true that getting your mapping caching attributes > out of sync can be a problem. Sure, but we're still stuck with needing lots of scatterlist list elements and needing to copy them to share physical buffers. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>