On Tue, Jul 16, 2019 at 05:13:10PM -0700, Rob Clark wrote: > On Tue, Jul 16, 2019 at 4:39 PM Eric Anholt <eric@xxxxxxxxxx> wrote: > > > > Rob Clark <robdclark@xxxxxxxxx> writes: > > > > > From: Rob Clark <robdclark@xxxxxxxxxxxx> > > > > > > Since there is no real device associated with VGEM, it is impossible to > > > end up with appropriate dev->dma_ops, meaning that we have no way to > > > invalidate the shmem pages allocated by VGEM. So, at least on platforms > > > without drm_cflush_pages(), we end up with corruption when cache lines > > > from previous usage of VGEM bo pages get evicted to memory. > > > > > > The only sane option is to use cached mappings. > > > > This may be an improvement, but... > > > > pin/unpin is only on attaching/closing the dma-buf, right? So, great, > > you flushed the cached map once after exporting the vgem dma-buf to the > > actual GPU device, but from then on you still have no interface for > > getting coherent access through VGEM's mapping again, which still > > exists. > > In *theory* one would detach before doing further CPU access to > buffer, and then re-attach when passing back to GPU. > > Ofc that isn't how actual drivers do things. But maybe it is enough > for vgem to serve it's purpose (ie. test code). > > > I feel like this is papering over something that's really just broken, > > and we should stop providing VGEM just because someone wants to write > > dma-buf test code without driver-specific BO alloc ioctl code. > > yup, it is vgem that is fundamentally broken (or maybe more > specifically doesn't fit in w/ dma-mappings view of how to do cache > maint), and I'm just papering over it because people and CI systems > want to be able to use it to do some dma-buf tests ;-) > > I'm kinda wondering, at least for arm/dt based systems, if there is a > way (other than in early boot) that we can inject a vgem device node > into the dtb. That isn't a thing drivers should normally do, but (if > possible) since vgem is really just test infrastructure, it could be a > way to make dma-mapping happily think vgem is a real device. Or we just extend drm_cflush_pages with the cflushing we need (at least for those arms where this is possible, let's ignore the others) and accept for a few more years that dma-api doesn't fit? Note this would need to be a full copypasta of what the arch code has (since just exporting the function was shot down before), but I really don't care about the resulting wailing if we do this. -Daniel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
