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Subject: Re: [PATCH 10/10] venus: dec: make decoder compliant with stateful
 codec API
To:     Stanimir Varbanov <stanimir.varbanov@linaro.org>,
        Tomasz Figa <tfiga@chromium.org>
Cc:     Linux Media Mailing List <linux-media@vger.kernel.org>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-arm-msm <linux-arm-msm@vger.kernel.org>,
        Vikash Garodia <vgarodia@codeaurora.org>,
        Alexandre Courbot <acourbot@chromium.org>,
        Malathi Gottam <mgottam@codeaurora.org>
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From:   Hans Verkuil <hverkuil@xs4all.nl>
Message-ID: <0cf25512-97b3-f46a-c266-508368e261d8@xs4all.nl>
Date:   Mon, 3 Jun 2019 09:25:51 +0200
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On 5/31/19 10:01 AM, Stanimir Varbanov wrote:
> Hi,
> 
> On 5/27/19 11:18 AM, Tomasz Figa wrote:
>> On Mon, May 27, 2019 at 4:39 PM Hans Verkuil <hverkuil@xs4all.nl> wrote:
>>>
>>> On 5/27/19 5:51 AM, Tomasz Figa wrote:
>>>> On Tue, May 21, 2019 at 9:27 PM Hans Verkuil <hverkuil@xs4all.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@linaro.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@linaro.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.
> 
> Unfortunately in Venus case that is not an easy task (as I tried to
> explain why above).
> 
> To have an unified and common code for all different SoCs and
> firmware/hardware versions I decided to set the minimum supported
> resolution for the decoder (no matter what the user said) and trigger
> the reconfiguration event always. Something more, I need the event also
> to retrieve the minimum capture buffers
> (V4L2_CID_MIN_BUFFERS_FOR_CAPTURE) and sizes for capture and
> internal/scratch buffers as well, thus I really need to wait for that
> event.
> 
> So, just to confirm - you are fine with blocking G_FMT (not REQBUF) when
> the user doesn't subscribe for v4l2 events?

'Fine' is too strong a word :-)

But I think this is a reasonable compromise.

Document carefully why you are doing this, since it is purely for backwards
compatibility reasons. And perhaps at some point in the future the workaround
might be removed again.

Regards,

	Hans

> 
>>
>>>
>>>> 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?
> 
> No it isn't.
>