[PATCH 7/7] drm/syncobj: use the timeline point in drm_syncobj_find_fence
Daniel Vetter
daniel at ffwll.ch
Fri Nov 23 12:26:37 UTC 2018
On Fri, Nov 23, 2018 at 12:02:41PM +0000, Koenig, Christian wrote:
> Am 23.11.18 um 12:03 schrieb Christian König:
> > Am 23.11.18 um 11:56 schrieb zhoucm1:
> >>
> >>
> >> On 2018年11月23日 18:10, Koenig, Christian wrote:
> >>> Am 23.11.18 um 03:36 schrieb zhoucm1:
> >>>>
> >>>> On 2018年11月22日 19:30, Christian König wrote:
> >>>>> Am 22.11.18 um 07:52 schrieb zhoucm1:
> >>>>>>
> >>>>>> On 2018年11月15日 19:12, Christian König wrote:
> >>>>>>> Implement finding the right timeline point in
> >>>>>>> drm_syncobj_find_fence.
> >>>>>>>
> >>>>>>> Signed-off-by: Christian König <christian.koenig at amd.com>
> >>>>>>> ---
> >>>>>>> drivers/gpu/drm/drm_syncobj.c | 10 +++++++++-
> >>>>>>> 1 file changed, 9 insertions(+), 1 deletion(-)
> >>>>>>>
> >>>>>>> diff --git a/drivers/gpu/drm/drm_syncobj.c
> >>>>>>> b/drivers/gpu/drm/drm_syncobj.c
> >>>>>>> index 589d884ccd58..d42c51520da4 100644
> >>>>>>> --- a/drivers/gpu/drm/drm_syncobj.c
> >>>>>>> +++ b/drivers/gpu/drm/drm_syncobj.c
> >>>>>>> @@ -307,9 +307,17 @@ int drm_syncobj_find_fence(struct drm_file
> >>>>>>> *file_private,
> >>>>>>> return -ENOENT;
> >>>>>>> *fence = drm_syncobj_fence_get(syncobj);
> >>>>>>> - if (!*fence) {
> >>>>>>> + if (!*fence)
> >>>>>>> ret = -EINVAL;
> >>>>>>> +
> >>>>>>> + if (!ret && point) {
> >>>>>>> + dma_fence_chain_for_each(*fence) {
> >>>>>>> + if (!to_dma_fence_chain(*fence) ||
> >>>>>>> + (*fence)->seqno <= point)
> >>>>>>> + break;
> >>>>>> This condition isn't enough to find proper point.
> >>>>>> For two examples:
> >>>>>> a. No garbage collection happens, the points in chain are
> >>>>>> 1----3----6----9----12---18---20, if user wants to get point17, then
> >>>>>> we should return node 18.
> >>>>> And that is exactly what's wrong in the original logic. In this case
> >>>>> we need to return 12, not 18 because point 17 could have already been
> >>>>> garbage collected.
> >>>> I don't think so, the 'a' case I already assume there isn't garbage
> >>>> collection. If user wants to get point17, then we should return
> >>>> node 18.
> >>>> timeline means point[N] must be signaled later than point[N-1].
> >>>> Point[12] just can make sure point[1] ~point[12] are signaled.
> >>>> Point[18] signal can make sure point[17] is signaled.
> >>>> So this case we need to return 18, not 12, which is key timeline
> >>>> concept.
> >>> No, exactly that's incorrect. When we ask for 17 and can't find it then
> >>> this means it either never existed or that it is signaled already.
> >>>
> >>> Returning a lower number in this case or even a stub fence is perfectly
> >>> fine since we only need to wait for that one in this case.
> >>>
> >>> If we return 18 in this case then we add incorrect synchronization when
> >>> there shouldn't be any.
> >> No, That will make timeline not work at all and break timeline
> >> semantics totally.
> >>
> >> If there aren't point18 and point20, the chain is
> >> 1----3----6----9----12, if user wants to get point 17, you also
> >> return 12? if yes, which absolutely is incorrect. The answer should
> >> be NO, right? point17 should be waited on there until a bigger point
> >> is coming.
> >
> > Correct, but this is a different case. In this situation we either
> > return an error or wait for point 17 (or something >=17) to show up.
> >
> > The key difference is if point 17 shows up then we return point 17,
> > but if point 18 shows up then we need to return point 12.
> >
> >> For chain is 1----3----6----9----12---18---20, if user wants to wait
> >> on any one of points 13,14,15,16,17,18, we must wait for point 18,
> >> this is timeline semantic.
> >
> > Ah, now I understand. You are still sticking with the assumption of a
> > fence number, right?
> >
> > In other words what you imply here is that we have the same semantic
> > as when somebody waits for a memory location to be written by number
> > 17, right? In this case the semantics you describe here indeed applies.
> >
> > But that is certainly not what we want to implement or otherwise we
> > will never be able to garbage collect the numbers in between.
> >
> > So if Vulkan has this requirement then we need to reject that.
>
> Backing of and reconsidering this I came to the conclusion that what you
> suggest here is actually the most defensive solution.
>
> In other words it is the solution where it's most likely that nothing
> goes wrong because the worst thing that can happen is that we
> synchronize to much, but never to less.
>
> Going to think about it how we can bring that into alignment with the
> proposed garbage collection.
Should we implement the tests first (either as in-kernel unit tests, like
we have some, or in igt on top of vgem), agree on the semantics we want,
then work on the implementation?
All these discussions and gotchas and "oops another corner case we missed"
when only looking at the implementation feels like it could work out
better if we attack this from the other side of the uapi barrier ...
Just a thought.
-Daniel
>
> Thanks,
> Christian.
>
> >
> > Regards,
> > Christian.
> >
> >>
> >> You can also check sw_sync.c for timeline meaning.
> >>
> >> -David
> >>>
> >>> Christian.
> >>>
> >>>> -David
> >>>>>> b. garbage collection happens on point6, chain would be updated to
> >>>>>> 1---3---9---12---18---20, if user wants to get point5, then we
> >>>>>> should return node 3, but if user wants to get point 7, then we
> >>>>>> should return node 9.
> >>>>> Why? That doesn't seem to make any sense to me.
> >>>>>
> >>>>>> I still have no idea how to satisfy all these requirements with your
> >>>>>> current chain-fence. All these logic just are same we encountered
> >>>>>> before, we're walking them again. After solving these problems, I
> >>>>>> guess all design is similar as before.
> >>>>>>
> >>>>>> In fact, I don't know what problem previous design has, maybe there
> >>>>>> are some bugs, can't we fix these bugs by time going? Who can make
> >>>>>> sure his implementation never have bugs?
> >>>>> Well there where numerous problems with the original design. For
> >>>>> example we need to reject the requirement that timeline fences are in
> >>>>> order because that doesn't make sense in the kernel.
> >>>>>
> >>>>> When userspace does something like submitting fences in the order 1,
> >>>>> 5, 3 then it is broken and can keep the pieces. In other words the
> >>>>> kernel should not care about that, but rather make sure that it never
> >>>>> looses any synchronization no matter what.
> >>>>>
> >>>>> Regards,
> >>>>> Christian.
> >>>>>
> >>>>>>
> >>>>>> -David
> >>>>>>> + }
> >>>>>>> }
> >>>>>>> +
> >>>>>>> drm_syncobj_put(syncobj);
> >>>>>>> return ret;
> >>>>>>> }
> >>
> >> _______________________________________________
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> >> dri-devel at lists.freedesktop.org
> >> https://lists.freedesktop.org/mailman/listinfo/dri-devel
> >
>
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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