On Wed, May 26, 2021 at 02:08:19PM +0100, Daniel Stone wrote: > Hey, > > On Wed, 26 May 2021 at 13:35, Daniel Vetter <daniel@xxxxxxxx> wrote: > > On Wed, May 26, 2021 at 1:09 PM Daniel Stone <daniel@xxxxxxxxxxxxx> wrote: > > > Yeah, I don't think there's any difference between shared and > > > exclusive wrt safety. The difference lies in, well, exclusive putting > > > a hard serialisation barrier between everything which comes before and > > > everything that comes after, and shared being more relaxed to allow > > > for reads to retire in parallel. > > > > > > As said below, I think there's a good argument for the latter once you > > > get out of the very straightforward uses. One of the arguments for > > > these ioctls is to eliminate oversync, but then the import ioctl > > > mandates oversync in the case where the consumer only does > > > non-destructive reads - which is the case for the vast majority of > > > users! > > > > Just wanted to comment on this: Right now we attach always attach a > > shared end-of-batch fence to every dma_resv. So reads are > > automatically and always synced. So in that sense having an explicit > > ioct to set the read fence is not really useful, since at most you > > just make everything worse. > > Are you saying that if a compositor imports a client-provided dmabuf > as an EGLImage to use as a source texture for its rendering, and then > provides it to VA-API or V4L2 to use as a media encode source (both > purely read-only ops), that these will both serialise against each > other? Like, my media decode job won't begin execution until the > composition read has fully retired? > > If so, a) good lord that hurts, and b) what are shared fences actually ... for? Shared is shared, I just meant to say that we always add the shared fence. So an explicit ioctl to add more shared fences is kinda pointless. So yeah on a good driver this will run in parallel. On a not-so-good driver (which currently includes amdgpu and panfrost) this will serialize, because those drivers don't have the concept of a non-exclusive fence for such shared buffers (amdgpu does not sync internally, but will sync as soon as it's cross-drm_file). -Daniel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
