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

[Rob Clark]

On Mon, Feb 27, 2023 at 1:34 AM Pekka Paalanen <ppaalanen@xxxxxxxxx> wrote:
>
> On Fri, 24 Feb 2023 11:44:53 -0800
> Rob Clark <robdclark@xxxxxxxxx> wrote:
>
> > 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.
> >
> > Userspace already has various (driver specific) debugfs/sysfs that it
> > can use if it wants to make it hotter and drain batteries faster, so
> > I'm not seeing a strong need to cater to the "turn it up to eleven"
> > crowd here.  And really your point feels like a good reason to _not_
> > document this as anything more than a hint.
>
> My point is that no matter what you say in documentation or leave
> unsaid, people can and will abuse this by the behaviour it provides
> anyway, like every other UABI.
>
> So why not just document what it is supposed to do? It cannot get any
> worse. Maybe you get lucky instead and people don't abuse it that much
> if they read the docs.
>
> E.g. can it affect GPU job scheduling, can it affect GPU clocks, can it
> affect power consumption, and so on.

I expect, potentially, all or any, or none of the above ;-)

I guess I could document it as such, just to preempt potential
complaints about broken spacebar-heater.  The question is, where?  I
could add something about fence deadline hints in dma-buf.rst, which
would I think be useful in general for driver writers.  But there
isn't really any existing documents other than headerdoc comments for
sync_file ioctls, or drm_syncobj related ioctls (the latter are really
just for mesa to use, so maybe that is ok).

>
> > Back in the real world, mobile games are already well aware of the fps
> > vs battery-life (and therefore gameplay) tradeoff.  But what is
> > missing is a way to inform the kernel of userspace's intentions, so
> > that gpu dvfs can make intelligent decisions.  This series is meant to
> > bridge that gap.
>
> Then document that. As long as you document it properly: what you
> expect it to be used for and how.
>
> Or if this is reserved strictly for Mesa drivers, then document that.

I expect the SET_DEADLINE ioctl to be useful to compositors, and maybe
a few other cases.  I'd like to use it in virglrenderer to bridge
guest deadline hints to host kernel, for example.

BR,
-R

> You can also stop CC'ing me if you don't want attention to UABI docs. I
> don't read dri-devel@ unless I'm explicitly CC'd nowadays.
>
>
> Thanks,
> pq