On Tue, Aug 11, 2015 at 11:24:58AM +0100, Chris Wilson wrote: > On Tue, Aug 11, 2015 at 12:12:08PM +0200, Daniel Vetter wrote: > > On Mon, Aug 10, 2015 at 09:16:46PM +0100, Chris Wilson wrote: > > > If the set of pages is being imported from another device, we cannot > > > assume that it is fully coherent with the CPU cache, so mark it as such. > > > However, if the source is the shared memory vgem allocator, we could > > > treat the buffer as being cached (so long as all parties agree in the > > > case the same buffer is shared between multiple devices) but as of > > > today, vgem cannot export dma-bufs. > > > > > > Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> > > > Cc: Daniel Vetter <daniel.vetter@xxxxxxxx> > > > > Yay for x86 dma api to refuse acknowleding that incoherent devices exist > > (since this should be done in the dma-ap/sg stuff imo for dma-bufs). > > > > But since x86 assumes that everything is coherent shouldn't we do the > > inverse and use snooping/cacheable always? > > Actually, the convention for PRIME is that transfer bos are uncached > because we have to be sure that any write makes it into the foriegn > pages, that applies to both GPU and CPU writes, precisely because the > target is *incoherent*. > > > That should be correct for everything modern at least, and for old agp > > crap (do we care even, is sharing possible there?) snooping should only > > result in a bit more overhead. > > > > Or where exactly does this blow up? > > Consider a kms_crc-esque test using a radeon/nouveau bo imported into > i915 and accessed using i915 ioctls (with the CRC testing on the radeon > scanout). Scanout on radeon must be in vram afaik, and that's a place i915 can't get at. I think that even holds true for the integrated radones (they just use stolen for vram then). The other way round also doesn't work I think since i915 can't change the caching policy for radoen/nouveau access if radeon/nouveau want to write directly to main memory. Otoh I think pcie transactions just snoop the cache and never put anything in there. I still think that everything we import and don't have a clue about should probably be treated as snooped conceptually. But practically who knows what's going on ... -Daniel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch _______________________________________________ Intel-gfx mailing list Intel-gfx@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/intel-gfx
