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. BR, -R _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
