linux-kernel - Re: [PATCH v3 00/15] V4L2 Explicit Synchronization support

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Date:   Fri, 13 Oct 2017 02:56:07 +0100
From:   Brian Starkey <brian.starkey@....com>
To:     Gustavo Padovan <gustavo.padovan@...labora.com>
Cc:     Gustavo Padovan <gustavo@...ovan.org>, linux-media@...r.kernel.org,
        Hans Verkuil <hverkuil@...all.nl>,
        Mauro Carvalho Chehab <mchehab@....samsung.com>,
        Shuah Khan <shuahkh@....samsung.com>,
        linux-kernel@...r.kernel.org, Jonathan.Chai@....com
Subject: Re: [PATCH v3 00/15] V4L2 Explicit Synchronization support

Hi,

On Wed, Oct 04, 2017 at 05:08:55PM -0300, Gustavo Padovan wrote:
>Hi Brian,
>
>On Mon, 2017-10-02 at 14:41 +0100, Brian Starkey wrote:
>> Hi Gustavo,
>>
>> On Thu, Sep 07, 2017 at 03:42:11PM -0300, Gustavo Padovan wrote:
>> > From: Gustavo Padovan <gustavo.padovan@...labora.com>
>> >
>> > Hi,
>> >
>> > Refer to the documentation on the first patch for the details. The
>> > previous
>> > iteration is here: https://www.mail-archive.com/linux-media@vger.ke
>> > rnel.org/msg118077.html
>> >
>> > The 2nd patch proposes an userspace API for fences, then on patch 3
>> > we
>> > prepare to the addition of in-fences in patch 4, by introducing the
>> > infrastructure on vb2 to wait on an in-fence signal before queueing
>> > the
>> > buffer in the driver.
>> >
>> > Patch 5 fix uvc v4l2 event handling and patch 6 configure q->dev
>> > for
>> > vivid drivers to enable to subscribe and dequeue events on it.
>> >
>> > Patches 7-9 enables support to notify BUF_QUEUED events, the event
>> > send
>> > to userspace the out-fence fd and the index of the buffer that was
>> > queued.
>> >
>> > Patches 10-11 add support to mark queues as ordered. Finally
>> > patches 12
>> > and 13 add more fence infrastructure to support out-fences, patch
>> > 13 exposes
>> > close_fd() and patch 14 adds support to out-fences.
>> >
>> > It only works for ordered queues for now, see open question at the
>> > end
>> > of the letter.
>> >
>> > Test tool can be found at:
>> > https://git.collabora.com/cgit/user/padovan/v4l2-test.git/
>> >
>> > Main Changes
>> > ------------
>> >
>> > * out-fences: change in behavior: the out-fence fd now comes out of
>> > the
>> > BUF_QUEUED event along with the buffer id.
>>
>> The more I think about this, the more unfortunate it seems.
>> Especially
>> for our use-case (m2m engine which sits in front of the display
>> processor to convert the format of some buffers), having to wait for
>> the in-fence to signal before we can get an out-fence removes a lot
>> of
>> the advantages of having fences at all.
>
>Does your m2m driver ensures ordering between the buffer queued to it?
>

I'm not so familiar with the code, how can I check that?

>>
>> Ideally, we'd like to queue up our m2m work (while the GPU is still
>> rendering that buffer, holding the in-fence), immediately get the
>> out-fence for the m2m work, and pass that to DRM as the in-fence for
>> display. With the current behaviour we need to wait in userspace
>> before we can pass the buffer to display.
>>
>> Wouldn't it be possible to enforce that the buffers aren't queued
>> out-of-order in VB2? An easy way might be to (in qbuf) set a buffer's
>> ->in_fence to be a fence_array of all the ->in_fences from the
>> buffers
>> before it in the queue (and its own). That would then naturally order
>> the enqueue-ing in the driver, and allow you to return the out-fence
>> immediately.
>>
>> This would also solve your output devices question from below - a
>> buffer can never get queued in the driver until all of the buffers
>> which were QBUF'd before it are queued in the driver.
>
>What you say makes sense, what this proposal lacks the most now is
>feedback regarding its usecases. We can create a control setting to
>enforce ordering in the queue, if it's set we create the fence arrays.
>For output devices this should be set by default.

Yeah that could work. I can see that in some cases queueing
out-of-order as the fences signal would be the right thing to do, so
makes sense to allow both.

Thanks,
-Brian

>
>Gustavo
>
>-- 
>Gustavo Padovan
>Principal Software Engineer
>Collabora Ltd.