On Tue, Nov 08, 2016 at 01:43:40PM +0100, Daniel Vetter wrote: > On Tue, Nov 08, 2016 at 11:45:51AM +0000, Chris Wilson wrote: > > On Tue, Nov 08, 2016 at 12:32:56PM +0100, Daniel Vetter wrote: > > > On Tue, Nov 08, 2016 at 10:35:08AM +0000, Chris Wilson wrote: > > > > On Tue, Nov 08, 2016 at 03:54:47PM +0900, Gustavo Padovan wrote: > > > > > From: Gustavo Padovan <gustavo.padovan@xxxxxxxxxxxxxxx> > > > > > > > > > > Hi, > > > > > > > > > > This is yet another version of the DRM fences patches. Please refer > > > > > to the cover letter[1] in a previous version to check for more details. > > > > > > > > Explicit fencing is not a superset of the implicit fences. The driver > > > > may be using implicit fences (on a reservation object) to serialise > > > > asynchronous operations wrt to each other (such as dispatching threads > > > > to flush cpu caches to memory, manipulating page tables and the like > > > > before the flip). Since the user doesn't know about these operations, > > > > they are not included in the explicit fence they provide, at which point > > > > we can't trust their fence to the exclusion of the implicit fences... > > > > > > My thoughts are that in atomic_check drivers just fill in the fence from > > > the reservation_object (i.e. the uapi implicit fencing part). If there's > > > any additional work that's queued up in ->prepare_fb then I guess the > > > driver needs to track that internally, but _only_ for kernel-internally > > > queued work. > > > > That's not a trivial task to work out which of the fence contexts within > > the reservation object are required and which are to be replaced by the > > explicit fence, esp. when you have to consider external fences. > > Hm, what kind of async kernel tasks are you thinking off? Atm I don't know > of anyone who does e.g. clflush through the gpu. And ttm bo placement > moves for display should be explicit enough that drivers will deal with > them correctly. At least that seems to have been the conclusion from the > long amdgpu thread. Now that we (i915) serialise on an reservation_object (obj->resv), we have floated ideas to use that to serialise async tasks (such as offloading the 100ms clflush to a (cpu) worker, a gpu task would pose a similar problem with a fence inserted that is not exposed to userspace). Also tempted to look at using async tasks + fences to do GTT updates but that is not a common pain point at the moment, and cases where it is the GTT thrashing itself is the issue. So how does i915 deal with ttm bo fences? -Chris -- Chris Wilson, Intel Open Source Technology Centre _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
