Date:   Thu, 13 Jun 2019 21:09:35 -0400
From:   Nicolas Dufresne <nicolas@...fresne.ca>
To:     Hans Verkuil <hverkuil-cisco@...all.nl>,
        linux-media@...r.kernel.org
Cc:     Tomasz Figa <tfiga@...omium.org>, linux-kernel@...r.kernel.org,
        Alexandre Courbot <acourbot@...omium.org>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Stanimir Varbanov <stanimir.varbanov@...aro.org>,
        Andrew-CT Chen <andrew-ct.chen@...iatek.com>,
        Tiffany Lin <tiffany.lin@...iatek.com>,
        Pawel Osciak <posciak@...omium.org>
Subject: Re: [PATCHv4 0/2] Document memory-to-memory video codec interfaces

Le jeudi 13 juin 2019 à 08:48 +0200, Hans Verkuil a écrit :
> On 6/3/19 1:28 PM, Hans Verkuil wrote:
> > Since Tomasz was very busy with other things, I've taken over this
> > patch series. This v4 includes his draft changes and additional changes
> > from me.
> > 
> > This series attempts to add the documentation of what was discussed
> > during Media Workshops at LinuxCon Europe 2012 in Barcelona and then
> > later Embedded Linux Conference Europe 2014 in Düsseldorf and then
> > eventually written down by Pawel Osciak and tweaked a bit by Chrome OS
> > video team (but mostly in a cosmetic way or making the document more
> > precise), during the several years of Chrome OS using the APIs in
> > production.
> > 
> > Note that most, if not all, of the API is already implemented in
> > existing mainline drivers, such as s5p-mfc or mtk-vcodec. Intention of
> > this series is just to formalize what we already have.
> > 
> > Thanks everyone for the huge amount of useful comments to previous
> > versions of this series. Much of the credits should go to Pawel Osciak
> > too, for writing most of the original text of the initial RFC.
> > 
> > This v4 incorporates all known comments (let me know if I missed
> > something!) and should be complete for the decoder.
> > 
> > For the encoder there are two remaining TODOs for the API:
> > 
> > 1) Setting the frame rate so bitrate control can make sense, since
> >    they need to know this information.
> > 
> >    Suggested solution: require support for ENUM_FRAMEINTERVALS for the
> >    coded pixelformats and S_PARM(OUTPUT). Open question: some drivers
> >    (mediatek, hva, coda) require S_PARM(OUTPUT), some (venus) allow both
> >    S_PARM(CAPTURE) and S_PARM(OUTPUT). I am inclined to allow both since
> >    this is not a CAPTURE vs OUTPUT thing, it is global to both queues.
> 
> Alternative proposal:
> 
> 1) Add support for fractions (struct v4l2_fract) as a control type:
>    V4L2_CTRL_TYPE_FRACT.
> 
> 2) Add a new V4L2_CID_MPEG_FRAME_INTERVAL control.

Is the MPEG namespace historical ? That might be confusing for users.

> 
> Encoders shall support this control.
> 
> 3) For backwards compatibility reasons encoder drivers still have to
> support G/S_PARM, but this can now be implemented by standard helpers
> that query this control. Drivers also have to implement ENUM_FRAMEINTERVALS.

That's won't be very friendly for UI generator like qv4l2. Support for
v4l2_fract as control should include a way to describe the supported
values of that control the usual way I think.

Also, long term, it would be nice to have two sets of frame rates. The
one that the HW can handle "real-time" and the one that can be used for
bitrate calculation. So staying away from ENUM_FRAMEINTERVALS for
bitrate configuration would be nicer.

> If the range of intervals is always the same regardless of the frame size,
> then a helper can be used that queries the min/max/step of the control, but
> if it is dependent on the frame size, then it has to be implemented in the
> driver itself.
> 
> I'm sticking to frame intervals instead of frame rates for the simple reason
> that that's what V4L2 has used since the beginning. I think it is too confusing
> to change this to a frame rate. This is just my opinion, though.

I suggested frame rate since this is what I saw implemented by HW
registers (if you think it's worth it, I can try and make a list).
Also, frame-interval steps are not compatible with frame-rate steps
(something that was raised through a venus driver bug) last year. Even
v4l2-ctl was displaying that in a very confusing way. Something as
simple as 1 to 30 fps cannot be exposed through ENU_FRAMEINTERVALS. You
are forced to expose the full fractional range of interval, from 1/30
to 1/1. For Venus it was not that much of a trouble, since its stores a
framerate as Q16..

> 
> I also chose to make this a codec control, not a generic user control: this
> value together with the bit rate control(s) determine the compression size,
> it does not determine the actual time it takes for the encoder to compress
> the raw frames. Hence it is really not the same thing as the frame interval

That's a good point.

> of a video capture device. If we want to use a control for that as well in
> the future as a replacement for G/S_PARM, then that should be a new control.
> And we would like need per-pad controls as well in order to implement that.
> Which is a lot more work.
> 
> Regards,
> 
> 	Hans

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