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Subject: Re: [PATCH v2 2/2] media: docs-rst: Document memory-to-memory video
 encoder interface
To:     Tomasz Figa <tfiga@chromium.org>
Cc:     Nicolas Dufresne <nicolas@ndufresne.ca>,
        Linux Media Mailing List <linux-media@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        =?UTF-8?B?UGF3ZcWCIE/Fm2NpYWs=?= <posciak@chromium.org>,
        Alexandre Courbot <acourbot@chromium.org>,
        Kamil Debski <kamil@wypas.org>,
        Andrzej Hajda <a.hajda@samsung.com>,
        Kyungmin Park <kyungmin.park@samsung.com>,
        Jeongtae Park <jtp.park@samsung.com>,
        Philipp Zabel <p.zabel@pengutronix.de>,
        Tiffany Lin <tiffany.lin@mediatek.com>,
        Andrew-CT Chen <andrew-ct.chen@mediatek.com>,
        Stanimir Varbanov <stanimir.varbanov@linaro.org>,
        Todor Tomov <todor.tomov@linaro.org>,
        Paul Kocialkowski <paul.kocialkowski@bootlin.com>,
        Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
        dave.stevenson@raspberrypi.org,
        Ezequiel Garcia <ezequiel@collabora.com>,
        Maxime Jourdan <maxi.jourdan@wanadoo.fr>
References: <20181022144901.113852-1-tfiga@chromium.org>
 <20181022144901.113852-3-tfiga@chromium.org>
 <4cd223f0-b09c-da07-f26c-3b3f7a8868d7@xs4all.nl>
 <5fb0f2db44ba7aa3788b61f2aa9a30d4f4984de5.camel@ndufresne.ca>
 <d853eb91-c05d-fb10-f154-bc24e4ebb89d@xs4all.nl>
 <CAAFQd5COddN-YosKyfBJ7n=qt40ONP=YEjBo5HQBOPGhs19h+g@mail.gmail.com>
From:   Hans Verkuil <hverkuil@xs4all.nl>
Message-ID: <fcad4ca0-cfdd-d0fb-4b18-808426584755@xs4all.nl>
Date:   Wed, 23 Jan 2019 12:28:54 +0100
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On 01/23/19 11:00, Tomasz Figa wrote:
> On Sat, Nov 17, 2018 at 8:37 PM Hans Verkuil <hverkuil@xs4all.nl> wrote:
>>
>> On 11/17/2018 05:18 AM, Nicolas Dufresne wrote:
>>> Le lundi 12 novembre 2018 à 14:23 +0100, Hans Verkuil a écrit :
>>>> On 10/22/2018 04:49 PM, Tomasz Figa wrote:
> [snip]
>>>>> +      rely on it. The ``V4L2_BUF_FLAG_LAST`` buffer flag should be used
>>>>> +      instead.
>>>>
>>>> Question: should new codec drivers still implement the EOS event?
>>>
>>> I'm been asking around, but I think here is a good place. Do we really
>>> need the FLAG_LAST in userspace ? Userspace can also wait for the first
>>> EPIPE return from DQBUF.
>>
>> I'm interested in hearing Tomasz' opinion. This flag is used already, so there
>> definitely is a backwards compatibility issue here.
>>
> 
> FWIW, it would add the overhead of 1 more system call, although I
> don't think it's of our concern.
> 
> My personal feeling is that using error codes for signaling normal
> conditions isn't very elegant, though.

I agree. Let's keep this flag.

Regards,

	Hans

> 
>>>
>>>>
>>>>> +
>>>>> +3. Once all ``OUTPUT`` buffers queued before the ``V4L2_ENC_CMD_STOP`` call and
>>>>> +   the last ``CAPTURE`` buffer are dequeued, the encoder is stopped and it will
>>>>> +   accept, but not process any newly queued ``OUTPUT`` buffers until the client
>>>>> +   issues any of the following operations:
>>>>> +
>>>>> +   * ``V4L2_ENC_CMD_START`` - the encoder will resume operation normally,
>>>>
>>>> Perhaps mention that this does not reset the encoder? It's not immediately clear
>>>> when reading this.
>>>
>>> Which drivers supports this ? I believe I tried with Exynos in the
>>> past, and that didn't work. How do we know if a driver supports this or
>>> not. Do we make it mandatory ? When it's not supported, it basically
>>> mean userspace need to cache and resend the header in userspace, and
>>> also need to skip to some sync point.
>>
>> Once we agree on the spec, then the next step will be to add good compliance
>> checks and update drivers that fail the tests.
>>
>> To check if the driver support this ioctl you can call VIDIOC_TRY_ENCODER_CMD
>> to see if the functionality is supported.
> 
> There is nothing here for the hardware to support. It's an entirely
> driver thing, since it just needs to wait for the encoder to complete
> all the pending frames and stop enqueuing more frames to the decoder
> until V4L2_ENC_CMD_START is called. Any driver that can't do it must
> be fixed, since otherwise you have no way to ensure that you got all
> the encoded output.
> 
> Best regards,
> Tomasz
>