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. Eric -- To unsubscribe from this list: send the line "unsubscribe linux-omap" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
