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

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Date:   Tue, 21 Feb 2023 11:48:35 -0500
From:   Luben Tuikov <luben.tuikov@....com>
To:     Rob Clark <robdclark@...il.com>,
        Pekka Paalanen <ppaalanen@...il.com>
Cc:     Rob Clark <robdclark@...omium.org>,
        Gustavo Padovan <gustavo@...ovan.org>,
        Tvrtko Ursulin <tvrtko.ursulin@...el.com>,
        Christian König <ckoenig.leichtzumerken@...il.com>,
        Michel Dänzer <michel@...nzer.net>,
        open list <linux-kernel@...r.kernel.org>,
        dri-devel@...ts.freedesktop.org,
        Christian König <christian.koenig@....com>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@...ts.linaro.org>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Alex Deucher <alexander.deucher@....com>,
        freedreno@...ts.freedesktop.org,
        Sumit Semwal <sumit.semwal@...aro.org>,
        "open list:SYNC FILE FRAMEWORK" <linux-media@...r.kernel.org>
Subject: Re: [PATCH v4 06/14] dma-buf/sync_file: Support (E)POLLPRI

On 2023-02-20 11:14, Rob Clark wrote:
> On Mon, Feb 20, 2023 at 12:53 AM Pekka Paalanen <ppaalanen@...il.com> wrote:
>>
>> On Sat, 18 Feb 2023 13:15:49 -0800
>> Rob Clark <robdclark@...il.com> wrote:
>>
>>> From: Rob Clark <robdclark@...omium.org>
>>>
>>> Allow userspace to use the EPOLLPRI/POLLPRI flag to indicate an urgent
>>> wait (as opposed to a "housekeeping" wait to know when to cleanup after
>>> some work has completed).  Usermode components of GPU driver stacks
>>> often poll() on fence fd's to know when it is safe to do things like
>>> free or reuse a buffer, but they can also poll() on a fence fd when
>>> waiting to read back results from the GPU.  The EPOLLPRI/POLLPRI flag
>>> lets the kernel differentiate these two cases.
>>>
>>> Signed-off-by: Rob Clark <robdclark@...omium.org>
>>
>> Hi,
>>
>> where would the UAPI documentation of this go?
>> It seems to be missing.
> 
> Good question, I am not sure.  The poll() man page has a description,
> but my usage doesn't fit that _exactly_ (but OTOH the description is a
> bit vague).
> 
>> If a Wayland compositor is polling application fences to know which
>> client buffer to use in its rendering, should the compositor poll with
>> PRI or not? If a compositor polls with PRI, then all fences from all
>> applications would always be PRI. Would that be harmful somehow or
>> would it be beneficial?
> 
> I think a compositor would rather use the deadline ioctl and then poll
> without PRI.  Otherwise you are giving an urgency signal to the fence
> signaller which might not necessarily be needed.
> 
> The places where I expect PRI to be useful is more in mesa (things
> like glFinish(), readpix, and other similar sorts of blocking APIs)
Hi,

Hmm, but then user-space could do the opposite, namely, submit work as usual--never 
using the SET_DEADLINE ioctl, and then at the end, poll using (E)POLLPRI. That seems
like a possible usage pattern, unintended--maybe, but possible. Do we want to discourage
this? Wouldn't SET_DEADLINE be enough? I mean, one can call SET_DEADLINE with the current
time, and then wouldn't that be equivalent to (E)POLLPRI?
-- 
Regards,
Luben