linux-kernel - Re: [PATCH v4 06/14] dma-buf/sync

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Date:   Fri, 24 Feb 2023 12:24:03 +0200
From:   Pekka Paalanen <ppaalanen@...il.com>
To:     Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>
Cc:     Rob Clark <robdclark@...il.com>,
        Rob Clark <robdclark@...omium.org>,
        Tvrtko Ursulin <tvrtko.ursulin@...el.com>,
        Gustavo Padovan <gustavo@...ovan.org>,
        Michel Dänzer <michel@...nzer.net>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        open list <linux-kernel@...r.kernel.org>,
        dri-devel@...ts.freedesktop.org,
        Sumit Semwal <sumit.semwal@...aro.org>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@...ts.linaro.org>,
        Luben Tuikov <luben.tuikov@....com>,
        Christian König <ckoenig.leichtzumerken@...il.com>,
        Alex Deucher <alexander.deucher@....com>,
        freedreno@...ts.freedesktop.org,
        Christian König <christian.koenig@....com>,
        "open list:SYNC FILE FRAMEWORK" <linux-media@...r.kernel.org>
Subject: Re: [PATCH v4 06/14] dma-buf/sync_file: Support (E)POLLPRI

On Fri, 24 Feb 2023 09:41:46 +0000
Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com> wrote:

> On 24/02/2023 09:26, Pekka Paalanen wrote:
> > On Thu, 23 Feb 2023 10:51:48 -0800
> > Rob Clark <robdclark@...il.com> wrote:
> >   
> >> On Thu, Feb 23, 2023 at 1:38 AM Pekka Paalanen <ppaalanen@...il.com> wrote:  
> >>>
> >>> On Wed, 22 Feb 2023 07:37:26 -0800
> >>> Rob Clark <robdclark@...il.com> wrote:
> >>>     
> >>>> On Wed, Feb 22, 2023 at 1:49 AM Pekka Paalanen <ppaalanen@...il.com> 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.

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

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