Re: [PATCH v2 2/2] media: docs-rst: Document memory-to-memory video encoder interface

[Tomasz Figa]

On Sun, Nov 18, 2018 at 10:34 AM Nicolas Dufresne <nicolas@xxxxxxxxxxxx> wrote:
>
> Le samedi 17 novembre 2018 Ã 12:37 +0100, Hans Verkuil a Ãcrit :
> > > > Does V4L2_CID_MIN_BUFFERS_FOR_CAPTURE make any sense for encoders?
> > >
> > > We do account for it in GStreamer (the capture/output handling is
> > > generic), but I don't know if it's being used anywhere.
> >
> > Do you use this value directly for REQBUFS, or do you use it as the minimum
> > value but in practice use more buffers?
>
> We add more buffers to that value. We assume this value is what will be
> held by the driver, hence without adding some buffers, the driver would
> go idle as soon as one is dequeued. We also need to allocate for the
> importing driver.
>
> In general, if we have a pipeline with Driver A sending to Driver B,
> both driver will require a certain amount of buffers to operate. E.g.
> with DRM display, the driver will hold on 1 buffer (the scannout
> buffer).
>
> In GStreamer, it's implemented generically, so we do:
>
>   MIN_BUFFERS_FOR + remote_min + 1
>
> If only MIN_BUFFERS_FOR was allocated, ignoring remote driver
> requirement, the streaming will likely get stuck.

What happens if the driver doesn't report it?

Best regards,
Tomasz