Re: How to get last frames in encode sequence returned by v4l2 encoder driver without V4L2_ENC_CMD_STOP

[Hirokazu Honda <hiroh@chromium.org>]

HI Alexandre,

On Tue, Nov 24, 2020 at 7:17 PM Alexandre Courbot <acourbot@chromium.org> wrote:
>
> Hi Hiro,
>
> On Fri, Nov 13, 2020 at 6:04 PM Hirokazu Honda <hiroh@chromium.org> wrote:
> >
> > Hi,
> >
> > According to the official v4l2 encoder driver usage description [1],
> > v4l2 steatful encoder driver doesn't have a guarantee when frames fed
> > to a driver will be returned.
> > To make sure all pending frames are output by the driver, an app must
> > call VIDIOC_ENCODER_CMD with cmd=V4L2_ENC_CMD_STOP.
> > However, it is not mandatory to support the command in the current
> > v4l2 stateful encoder API specification.
> > An app can check it by VIDIOC_TRY_ENCODER_CMD beforehand.
> > My question is when an app has to get all the frames of an encoder
> > sequence, how we can achieve this without V4L2_ENC_CMD_STOP support.
> > This demand is natural and in fact WebCodecs [2] requires this.
> >
> > I think there are two options,
> > 1.) Ensure that a driver will eventually output frames if it doesn't
> > support V4L2_ENC_CMD_STOP.
> > 2.) Change V4L2_ENC_CMD_STOP support to be mandatory
>
> Unless I am missing the part of the spec that says the contrary,
> V4L2_ENC_CMD_STOP is part of the encoder specification, and thus is
> mandatory. Some older drivers might not have support for it, in such
> cases the correct course of action would be to fix them.
>

I researched the API documents.
The statement that the support is mandatory to stateful encoders is
added from the latest document v5.9 [1],
It states optional in the API document of v4.19 and v5.8.
Hence my question is obsolete.

[1] https://www.kernel.org/doc/html/v5.9/userspace-api/media/v4l/vidioc-encoder-cmd.html
[2] https://www.kernel.org/doc/html/v4.19/media/uapi/v4l/vidioc-encoder-cmd.html
[3] https://www.kernel.org/doc/html/v5.8/userspace-api/media/v4l/vidioc-encoder-cmd.html?highlight=v4l2_enc_cmd_stop

Best Regards,
-Hiro
> >
> > Any comments are appreciated.
> > Thanks so much in advance.
> >
> > [1] https://www.kernel.org/doc/html/latest/userspace-api/media/v4l/dev-encoder.html#drain
> > [2] https://web.dev/webcodecs/
> >
> > Sincerely,
> > -Hiro