Re: [PATCH v4 06/14] dma-buf/sync_file: Support (E)POLLPRI

[Rob Clark]

On Fri, Feb 24, 2023 at 2:24 AM Pekka Paalanen <ppaalanen@xxxxxxxxx> wrote:
>
> On Fri, 24 Feb 2023 09:41:46 +0000
> Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx> wrote:
>
> > On 24/02/2023 09:26, Pekka Paalanen wrote:
> > > On Thu, 23 Feb 2023 10:51:48 -0800
> > > Rob Clark <robdclark@xxxxxxxxx> wrote:
> > >
> > >> On Thu, Feb 23, 2023 at 1:38 AM Pekka Paalanen <ppaalanen@xxxxxxxxx> wrote:
> > >>>
> > >>> On Wed, 22 Feb 2023 07:37:26 -0800
> > >>> Rob Clark <robdclark@xxxxxxxxx> wrote:
> > >>>
> > >>>> On Wed, Feb 22, 2023 at 1:49 AM Pekka Paalanen <ppaalanen@xxxxxxxxx> wrote:
> > >
> > > ...
> > >
> > >>>>> On another matter, if the application uses SET_DEADLINE with one
> > >>>>> timestamp, and the compositor uses SET_DEADLINE on the same thing with
> > >>>>> another timestamp, what should happen?
> > >>>>
> > >>>> The expectation is that many deadline hints can be set on a fence.
> > >>>> The fence signaller should track the soonest deadline.
> > >>>
> > >>> You need to document that as UAPI, since it is observable to userspace.
> > >>> It would be bad if drivers or subsystems would differ in behaviour.
> > >>>
> > >>
> > >> It is in the end a hint.  It is about giving the driver more
> > >> information so that it can make better choices.  But the driver is
> > >> even free to ignore it.  So maybe "expectation" is too strong of a
> > >> word.  Rather, any other behavior doesn't really make sense.  But it
> > >> could end up being dictated by how the hw and/or fw works.
> > >
> > > It will stop being a hint once it has been implemented and used in the
> > > wild long enough. The kernel userspace regression rules make sure of
> > > that.
> >
> > Yeah, tricky and maybe a gray area in this case. I think we eluded
> > elsewhere in the thread that renaming the thing might be an option.
> >
> > So maybe instead of deadline, which is a very strong word, use something
> > along the lines of "present time hint", or "signalled time hint"? Maybe
> > reads clumsy. Just throwing some ideas for a start.
>
> You can try, but I fear that if it ever changes behaviour and
> someone notices that, it's labelled as a kernel regression. I don't
> think documentation has ever been the authoritative definition of UABI
> in Linux, it just guides drivers and userspace towards a common
> understanding and common usage patterns.
>
> So even if the UABI contract is not documented (ugh), you need to be
> prepared to set the UABI contract through kernel implementation.
>
> If you do not document the UABI contract, then different drivers are
> likely to implement it differently, leading to differing behaviour.
> Also userspace will invent wild ways to abuse the UABI if there is no
> documentation guiding it on proper use. If userspace or end users
> observe different behaviour, that's bad even if it's not a regression.
>
> I don't like the situation either, but it is what it is. UABI stability
> trumps everything regardless of whether it was documented or not.
>
> I bet userspace is going to use this as a "make it faster, make it
> hotter" button. I would not be surprised if someone wrote a LD_PRELOAD
> library that stamps any and all fences with an expired deadline to
> just squeeze out a little more through some weird side-effect.

Ok, maybe we can rename the SET_DEADLINE ioctl to SPACEBAR_HEATER ;-)

BR,
-R

> Well, that's hopefully overboard in scaring, but in the end, I would
> like to see UABI documented so I can have a feeling of what it is for
> and how it was intended to be used. That's all.
>
>
> Thanks,
> pq