On Wed, Aug 19, 2015 at 03:53:44PM +0200, Tobias Jakobi wrote: > Adding Jérôme to Cc. I think he looked the userptr code before, so maybe > he has some idea what is going wrong here. > > I also had a look at the code, but my knowledge about the DMA API is > almost nonexistant. However I can see that before doing any DMA via the > G2D on the buffer the code calls dma_map() on it, and also unmaps it > when the commandlist is finished. > > > With best wishes, > Tobias > > > Tobias Jakobi wrote: > > Thanks Lucas for the explanation! > > > > > > Lucas Stach wrote: > >> Hi Tobias, > >> > >> Am Sonntag, den 16.08.2015, 14:48 +0200 schrieb Tobias Jakobi: > >>> Hello, > >>> > >>> some time ago I checked whether I could use the userptr functionality to > >>> do zero-copy from userspace allocated buffers via the G2D. This didn't > >>> work out so well, so kinda put this to the bottom of my TODO list. > >>> > >>> Now that IOMMU support has landed and Jan Kara has rewrote page pinning > >>> using frame vectors (see [1]) I gave userptr another try. > >>> > >>> The results are much better. I'm not experiencing any kernel lockups or > >>> sysmmu pagefaults anymore. However the image now suffers from visual > >>> artifacts. These images show the nature of the artifacts: > >>> http://i.imgur.com/nzT6g3Y.jpg > >>> http://i.imgur.com/wkuYI6X.jpg > >>> > >>> The corruption always manifests itself in these pixel lines of fixed > >>> size and wrong color. > >>> > >>> I have written a testcase as part of libdrm for this issue: > >>> https://github.com/tobiasjakobi/libdrm/commit/db8bf6844436598251f67a71fc334b929bfb2b71 > >>> > >>> It allocates N (N an even number) buffers which are aligned to the > >>> system pagesize. Then it does this each iteration: > >>> 1) Fill the first N/2 buffers with random data > >>> 2) Copy the first half to the second half of the buffers > >>> 3) memcmp() first and second half (verification pass) > >>> > >>> Usually this verification already fails on the first iteration. An > >>> interesting observation is that increasing (!) the buffer size (so the > >>> amount of pixels that have to copied per buffer grows) makes this issue > >>> less likely to happen. > >>> > >>> With the default 512x512 buffers however it happens, like I said above, > >>> almost immediately. > >>> > >> This is obviously a cache flush missing. The memory you get from > >> userspace is normal cached memory, so to make it visible to the GPU you > >> need to flush parts of the cache out to main memory. > >> > >> The corruption you are seeing is just unflushed cachelines. This also > >> explains why increasing the buffer size helps: the more memory the CPU > >> touches the more cachelines will be flushed out to be replaced with new > >> data. > > I should point out that the snapshots I uploaded were done with a > > different setup. There only the source memory of the G2D operation is a > > userspace allocated buffer. The destination is a GEM buffer allocated > > through libdrm, which is then used as framebuffer. So the issue already > > appears when just the source is userspace allocated. > > This is still consistent with cachelines issue. Is your GPU & IOMMU cache coherent with the CPU ? If not then it means you need to cache flush the buffer before you use it with the GPU. The dma API provide few helpers for that. Cheers, Jérôme _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/dri-devel
