On Tue, Mar 23, 2021 at 11:23 AM Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx> wrote: > > > On 23/03/2021 13:23, Daniel Vetter wrote: > > On Tue, Mar 23, 2021 at 09:14:36AM +0000, Tvrtko Ursulin wrote: > >> > >> On 22/03/2021 16:43, Daniel Vetter wrote: > >>> On Mon, Mar 22, 2021 at 4:31 PM Tvrtko Ursulin > >>> <tvrtko.ursulin@xxxxxxxxxxxxxxx> wrote: > >>>> > >>>> > >>>> On 22/03/2021 14:57, Daniel Vetter wrote: > >>>>> On Mon, Mar 22, 2021 at 3:33 PM Tvrtko Ursulin > >>>>> <tvrtko.ursulin@xxxxxxxxxxxxxxx> wrote: > >>>>>> > >>>>>> > >>>>>> On 22/03/2021 14:09, Daniel Vetter wrote: > >>>>>>> On Mon, Mar 22, 2021 at 11:22:01AM +0000, Tvrtko Ursulin wrote: > >>>>>>>> > >>>>>>>> On 19/03/2021 22:38, Jason Ekstrand wrote: > >>>>>>>>> This API allows one context to grab bits out of another context upon > >>>>>>>>> creation. It can be used as a short-cut for setparam(getparam()) for > >>>>>>>>> things like I915_CONTEXT_PARAM_VM. However, it's never been used by any > >>>>>>>>> real userspace. It's used by a few IGT tests and that's it. Since it > >>>>>>>>> doesn't add any real value (most of the stuff you can CLONE you can copy > >>>>>>>>> in other ways), drop it. > >>>>>>>> > >>>>>>>> No complaints to remove if it ended up unused outside IGT. Latter is a _big_ > >>>>>>>> problem though, since it is much more that a few IGT tests. So I really > >>>>>>>> think there really needs to be an evaluation and a plan for that (we don't > >>>>>>>> want to lose 50% of the coverage over night). > >>>>>>>> > >>>>>>>>> There is one thing that this API allows you to clone which you cannot > >>>>>>>>> clone via getparam/setparam: timelines. However, timelines are an > >>>>>>>>> implementation detail of i915 and not really something that needs to be > >>>>>>>> > >>>>>>>> Not really true timelines are i915 implementation detail. They are in fact a > >>>>>>>> dma-fence context:seqno concept, nothing more that than. I think you are > >>>>>>>> probably confusing struct intel_timeline with the timeline wording in the > >>>>>>>> uapi. Former is i915 implementation detail, but context:seqno are truly > >>>>>>>> userspace timelines. > >>>>>>> > >>>>>>> I think you're both saying the same thing and talking a bit past each > >>>>>>> another. > >>>>>>> > >>>>>>> Yes the timeline is just a string of dma_fence, that's correct. Now > >>>>>>> usually if you submit batches with execbuf, we have 3 ways to synchronize > >>>>>>> concurrent submission: implicit sync, sync_file and drm_syncob. They all > >>>>>>> map to different needs in different protocols/render apis. > >>>>>>> > >>>>>>> Now in one additional case the kernel makes sure that batchbuffers are > >>>>>>> ordered, and that's when you submit them to the same hw ctx. Because > >>>>>>> there's only 1 hw context and you really can't have batchbuffers run on > >>>>>>> that single hw context out of order. That's what the timeline object we > >>>>>>> talk about here is. But that largely is an internal implementation detail, > >>>>>>> which happens to also use most/all the same infrastructure as the > >>>>>>> dma_fence uapi pieces above. > >>>>>>> > >>>>>>> Now the internal implementation detail leaking here is that we exposed > >>>>>>> this to userspace, without there being any need for this. What Jason > >>>>>>> implements with syncobj in the next patch is essentially what userspace > >>>>>>> should have been using for cross-engine sync. media userspace doesn't care > >>>>>>> about interop with winsys/client apis, so they equally could have used > >>>>>>> implicit sync or sync_file here (which I think is the solution now for the > >>>>>>> new uapi prepped internally), since they all are about equally powerful > >>>>>>> for stringing batchbuffers together. > >>>>>> > >>>>>> Are you saying we exposed a single timeline of execution per hw context > >>>>>> via the single timeline flag?! > >>>>> > >>>>> Nope. > >>>>> > >>>>>> Timelines of execution were always exposed. Any "engine" (ring > >>>>>> previously) in I915_EXEC_RING_MASK was a single timeline of execution. > >>>>>> It is completely the same with engine map engines, which are also > >>>>>> different indices into I915_EXEC_RING_MASK space. > >>>>>> > >>>>>> Userspace was aware of these timelines forever as well. Media was > >>>>>> creating multiple contexts to have multiple timelines (so parallelism). > >>>>>> Everyone knew that engine-hopping submissions needs to be either > >>>>>> implicitly or explicitly synchronised, etc. > >>>>> > >>>>> Yup, I think we're saying the same thing here. > >>>>> > >>>>>> So I really don't see that we have leaked timelines as a concept *now*. > >>>>>> What the patch has exposed to userspace is a new way to sync between > >>>>>> timelines and nothing more. > >>>>> > >>>>> We've leaked it as something you can now share across hw context. > >>>> > >>>> Okay so we agree on most things but apparently have different > >>>> definitions of what it means to leak internal implementation details. > >>>> > >>>> While at the same time proof that we haven't leaked the internal > >>>> implementation details is that Jason was able to implement the single > >>>> timeline flag with a drm syncobj at the execbuf top level. (Well mostly, > >>>> ignoring the probably inconsequential difference of one vs multiple > >>>> fence contexts.) > >>> > >>> It's not a matching implementation. It's only good enough for what > >>> media needs, and essentially what media should have done to begin > >>> with. > >>> > >>> There's substantially different behaviour between SINGLE_TIMELINE and > >>> what Jason has done here when you race concurrent execbuf calls: > >>> Former guarantees total ordering, the latter doesn't even try. They > >>> are not the same thing, but luckily userspace doesn't care about that > >>> difference. > >> > >> Sounds like a very important difference to stress in the commit message. > >> > >> Secondly, I am unclear whether we have agreement on whether the single > >> timeline flag is leaking implementation details of the execlists scheduler > >> to userspace or not? > > > > I do think Jason&me agree on that it does leak an internal concept to > > userspace that we shouldn't leak. > > > > I'm honestly not entirely understanding your argument for why > > single_timeline isn't an internal concept somehow, and how exposing it to > > userspace doesn't leak that concept to userspace. Whether internally that > > concept is now perfectly represented by just struct intel_timeline, or > > maybe more the seqno/hswp, or more diffused through the code doesn't > > really change that we have an internal concept that we're now exposing for > > sharing in ways that wasn't possible before. > > Don't know, obviously we think with very different paradigms. > > GEM context always had as many timelines as there are engines in it's > map so multiple timelines is the default mode everyone is aware of. > > Single timeline flag added a new mode where instead of multiple > timelines single GEM context becomes a single timeline. > > The fact that userspace can achieve the single timeline execution on its > own should be an argument enough that it is not a new concept that got > leaked out. Definitely not any backend specific implementation details. > It simply added a new feature which may or may not have been needed. I just commented on the SINGLE_TIMELINE patch and will send the v3 momentarily. I think you'll find the commit message much more to your liking. :-) --Jason > Regards, > > Tvrtko > > P.S. > Or rename the flag in your mind to "I915_GEM_CONTEXT_SERIAL_EXECUTION" > or something and see if that still leaks the timeline or some > implementation details. > > P.P.S Keep in mind I am arguing on wording in single timeline flag > removal. Removal of timeline cloning is not controversial. _______________________________________________ Intel-gfx mailing list Intel-gfx@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/intel-gfx
