Date:   Mon, 27 May 2019 17:18:39 +0900
From:   Tomasz Figa <tfiga@...omium.org>
To:     Hans Verkuil <hverkuil@...all.nl>
Cc:     Stanimir Varbanov <stanimir.varbanov@...aro.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Vikash Garodia <vgarodia@...eaurora.org>,
        Alexandre Courbot <acourbot@...omium.org>,
        Malathi Gottam <mgottam@...eaurora.org>
Subject: Re: [PATCH 10/10] venus: dec: make decoder compliant with stateful
 codec API

On Mon, May 27, 2019 at 4:39 PM Hans Verkuil <hverkuil@...all.nl> wrote:
>
> On 5/27/19 5:51 AM, Tomasz Figa wrote:
> > On Tue, May 21, 2019 at 9:27 PM Hans Verkuil <hverkuil@...all.nl> wrote:
> >>
> >> On 5/21/19 11:09 AM, Tomasz Figa wrote:
> >>> Hi Stan,
> >>>
> >>> On Mon, May 20, 2019 at 11:47 PM Stanimir Varbanov
> >>> <stanimir.varbanov@...aro.org> wrote:
> >>>>
> >>>> Hi Tomasz,
> >>>>
> >>>> On 4/24/19 3:39 PM, Tomasz Figa wrote:
> >>>>> On Wed, Apr 24, 2019 at 9:15 PM Stanimir Varbanov
> >>>>> <stanimir.varbanov@...aro.org> wrote:
> >>>>>>
> >>>>>> Hi Hans,
> >>>>>>
> >>>>>> On 2/15/19 3:44 PM, Hans Verkuil wrote:
> >>>>>>> Hi Stanimir,
> >>>>>>>
> >>>>>>> I never paid much attention to this patch series since others were busy
> >>>>>>> discussing it and I had a lot of other things on my plate, but then I heard
> >>>>>>> that this patch made G_FMT blocking.
> >>>>>>
> >>>>>> OK, another option could be to block REQBUF(CAPTURE) until event from hw
> >>>>>> is received that the stream is parsed and the resolution is correctly
> >>>>>> set by application. Just to note that I'd think to this like a temporal
> >>>>>> solution until gstreamer implements v4l events.
> >>>>>>
> >>>>>> Is that looks good to you?
> >>>>>
> >>>>> Hmm, I thought we concluded that gstreamer sets the width and height
> >>>>> in OUTPUT queue before querying the CAPTURE queue and so making the
> >>>>> driver calculate the CAPTURE format based on what's set on OUTPUT
> >>>>> would work fine. Did I miss something?
> >>>>
> >>>> Nobody is miss something.
> >>>>
> >>>> First some background about how Venus implements stateful codec API.
> >>>>
> >>>> The Venus firmware can generate two events "sufficient" and
> >>>> "insufficient" buffer requirements (this includes decoder output buffer
> >>>> size and internal/scratch buffer sizes). Presently I always set minimum
> >>>> possible decoder resolution no matter what the user said, and by that
> >>>> way I'm sure that "insufficient" event will always be triggered by the
> >>>> firmware (the other reason to take this path is because this is the
> >>>> least-common-divider for all supported Venus hw/fw versions thus common
> >>>> code in the driver). The reconfiguration (during codec Initialization
> >>>> sequence) is made from STREAMON(CAPTURE) context. Now, to make that
> >>>> re-configuration happen I need to wait for "insufficient" event from
> >>>> firmware in order to know the real coded resolution.
> >>>>
> >>>> In the case of gstreamer where v4l2_events support is missing I have to
> >>>> block (wait for firmware event) REQBUF(CAPTURE) (vb2::queue_setup) or
> >>>> STREAMON(CAPTURE) (vb2::start_streaming).
> >>>>
> >>>> I tried to set the coded resolution to the firmware as-is it set by
> >>>> gstreamer but then I cannot receive the "sufficient" event for VP8 and
> >>>> VP9 codecs. So I return back to the solution with minimum resolution above.
> >>>>
> >>>> I'm open for suggestions.
> >>>
> >>> I think you could still keep setting the minimum size and wait for the
> >>> "insufficient" event. At the same time, you could speculatively
> >>> advertise the expected "sufficient" size on the CAPTURE queue before
> >>> the hardware signals those. Even if you mispredict them, you'll get
> >>> the event, update the CAPTURE resolution and send the source change
> >>> event to the application, which would then give you the correct
> >>> buffers. Would that work for you?
> >>
> >> As I understand it this still would require event support, which gstreamer
> >> doesn't have.
> >
> > I don't think it matches what I remember from the earlier discussion.
> > As long as Gstreamer sets the visible resolution (from the container
> > AFAIR) on OUTPUT, the driver would adjust it to something that is
> > expected to be the right framebuffer resolution and so Gstreamer would
> > be able to continue. Of course if the expected value doesn't match, it
> > wouldn't work, but it's the same as currently for Coda AFAICT.
> >
> >>
> >> I think it is OK to have REQBUFS sleep in this case. However, I would only
> >
> > Why REQBUFS? While that could possibly allow us to allocate the right
> > buffers, Gstreamer wouldn't be able to know the right format, because
> > it would query it before REQBUFS, wouldn't it?
>
> Oops, you are right. It's got to be in G_FMT(CAPTURE), but *only* if
> nobody subscribed to the SOURCE_CHANGE event.
>
> >
> > For this reason, s5p-mfc makes G_FMT(CAPTURE) blocking and if we
> > decide to forcefully keep the compatibility, even with in drivers, we
> > should probably do the same here.
> >
> >> enable this behavior if the application didn't subscribe to the SOURCE_CHANGE
> >> event. That's easy enough to check in the driver. And that means that if the
> >> application is well written, then the driver will behave in a completely
> >> standard way that the compliance test can check.
> >
> > I guess one could have some helpers for this. They would listen to the
> > source change events internally and block / wake-up appropriate ioctls
> > whenever necessary.
>
> I really do not want this for new drivers. gstreamer should be fixed.
> A blocking G_FMT is just plain bad. Only those drivers that do this, can
> still block if nobody subscribed to EVENT_SOURCE_CHANGE.

Yeah and that's why I just suggested to mimic coda, which doesn't
block, but apparently gstreamer still works with it.

>
> > Another question: If we intend this to be implemented in new drivers
> > too, should it be documented in the spec?
>
> We most certainly do NOT want to implement this in new drivers.
>

Makes sense.

When venus was merged initially, did it already have a blocking G_FMT?

Best regards,
Tomasz

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