On Thu, Jan 18, 2018 at 09:23:31AM -0800, Gurchetan Singh wrote: > On Wed, Jan 17, 2018 at 11:38 PM, Daniel Vetter <daniel@xxxxxxxx> wrote: > > > On Wed, Jan 17, 2018 at 11:49 PM, Gurchetan Singh > > <gurchetansingh@xxxxxxxxxxxx> wrote: > > > > > > On Wed, Jan 17, 2018 at 12:39 AM, Daniel Vetter <daniel@xxxxxxxx> wrote: > > >> > > >> On Tue, Jan 16, 2018 at 04:35:59PM -0800, Gurchetan Singh wrote: > > >> > This is required to use buffers allocated by vgem on AMD and ARM > > >> > devices. > > >> > We're experiencing a case where eviction of the cache races with > > >> > userspace > > >> > writes. To fix this, flush the cache after retrieving a page. > > >> > > > >> > Signed-off-by: Gurchetan Singh <gurchetansingh@xxxxxxxxxxxx> > > >> > --- > > >> > drivers/gpu/drm/vgem/vgem_drv.c | 1 + > > >> > 1 file changed, 1 insertion(+) > > >> > > > >> > diff --git a/drivers/gpu/drm/vgem/vgem_drv.c > > >> > b/drivers/gpu/drm/vgem/vgem_drv.c > > >> > index 35bfdfb746a7..fb263969f02d 100644 > > >> > --- a/drivers/gpu/drm/vgem/vgem_drv.c > > >> > +++ b/drivers/gpu/drm/vgem/vgem_drv.c > > >> > @@ -112,6 +112,7 @@ static int vgem_gem_fault(struct vm_fault *vmf) > > >> > break; > > >> > } > > >> > > > >> > + drm_flush_pages(obj->base.dev->dev, &page, 1); > > >> > > >> Uh ... what exactly are you doing? > > >> > > >> Asking because the entire "who's responsible for coherency" story is > > >> entirely undefined still when doing buffer sharing :-/ What is clear is > > >> that currently vgem entirely ignores this (there's not > > >> begin/end_cpu_access callback), mostly because the shared dma-buf > > support > > >> in drm_prime.c also entirely ignores this. > > > > > > > > > > > > This patch isn't trying to address the case of a dma-buf imported into > > vgem. > > > It's trying to address the case when a buffer is created by > > > vgem_gem_dumb_create, mapped by vgem_gem_dumb_map and then accessed by > > user > > > space. Since the page retrieved by shmem_read_mapping_page during the > > page > > > fault may still be in the cache, we're experiencing incorrect data in > > > buffer. Here's the test case we're running: > > > > > > https://chromium.googlesource.com/chromiumos/platform/drm-te > > sts/+/master/vgem_test.c > > > > 404s over here (Internal url?). > > > Hmm ... I was able to access that link without being logged in to any > accounts. > > > It fails on line 210 on AMD and ARM devices (not Intel though). > > > > So you _do_ import it on the other device driver as a dma-buf (and > > export it from vgem)? > > > Those portions of the test work fine (if the platform has a drm_clflush > implementation). vgem_prime_pin calls drm_clflush_pages for the exporting > case. Typically, ARM drivers flush the cache after drm_gem_get_pages() and > only do WC mappings, so import works. For Intel, there is some hardware > level coherency involved. The problem is vgem doesn't flush the cache on > ARM/AMD when getting pages for the non-export/non-import case (when > faulting after a vgem_gem_dumb_map, not during dma-buf mmap) -- i.e, during > regular use of the buffer. So if the idea is that the vgem buffers should be accessed using WC, then we need to switch the dump_map stuff to do wc. Flushing once is not going to fix things if you write again (afaik CrOS only does write-once and then throws buffers away again, but not sure). The other option is to throw dumb_map into the wind and only support dma-buf mmaping, which has a special ioctl for range flushing (so that we could flush before/after each access as needed). A gross hack would be to keep using dumb_map but abuse the dma-buf flushing ioctl for the flushing. The problem ofc is that there's no agreement between importers/exporters on who should flush when and where. Fixing that is way too much work, so personally I think the simplest clean fix is something along the lines of using the dma-buf flush ioctl (DMA_BUF_IOCTL_SYNC). Cheers, Daniel > > > > > >> And doing a one-time only > > >> flushing in your fault handler is definitely not going to fix this (at > > >> least not if you do anything else than one-shot uploads). > > > > > > > > > There used to be a be vgem_gem_get_pages function, but that's been > > removed. > > > I don't know where else to flush in this situation. > > > > dma_buf begin/end cpu access. Even exposed as an ioctl when you're > > using the dma-buf mmap stuff. Vgem doesn't have that, which means > > as-is the dumb mmap support of vgem can't really support this if you > > want to do explicit flushing. > > > > What would work is uncached/writecombine/coherent dma memory. But then > > we're in the middle of the entire > > "who's responsible. > > -Daniel > > > > > > > >> > > >> -Daniel > > >> > > >> > } > > >> > return ret; > > >> > } > > >> > -- > > >> > 2.13.5 > > >> > > > >> > _______________________________________________ > > >> > dri-devel mailing list > > >> > dri-devel@xxxxxxxxxxxxxxxxxxxxx > > >> > https://lists.freedesktop.org/mailman/listinfo/dri-devel > > >> > > >> -- > > >> Daniel Vetter > > >> Software Engineer, Intel Corporation > > >> http://blog.ffwll.ch > > > > > > > > > > > > > > -- > > Daniel Vetter > > Software Engineer, Intel Corporation > > +41 (0) 79 365 57 48 - http://blog.ffwll.ch > > -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch -- To unsubscribe from this list: send the line "unsubscribe linux-tegra" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
