linux-kernel - Re: [PATCH] media: vivid: Support 480p for webcam capture

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Date:   Mon, 8 Oct 2018 14:03:02 -0300
From:   Mauro Carvalho Chehab <mchehab+samsung@...nel.org>
To:     Kieran Bingham <kieran.bingham@...asonboard.com>
Cc:     Keiichi Watanabe <keiichiw@...omium.org>,
        linux-media@...r.kernel.org, Hans Verkuil <hverkuil@...all.nl>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-kernel@...r.kernel.org, tfiga@...omium.org,
        jcliang@...omium.org, shik@...omium.org
Subject: Re: [PATCH] media: vivid: Support 480p for webcam capture

Em Wed, 3 Oct 2018 12:14:22 +0100
Kieran Bingham <kieran.bingham@...asonboard.com> escreveu:

> > @@ -75,6 +76,8 @@ static const struct v4l2_fract webcam_intervals[VIVID_WEBCAM_IVALS] = {
> >  	{  1, 5 },
> >  	{  1, 10 },
> >  	{  1, 15 },
> > +	{  1, 15 },
> > +	{  1, 25 },  

As the code requires that VIVID_WEBCAM_IVALS would be twice the number
of resolutions, I understand why you're doing that.

> But won't this add duplicates of 25 and 15 FPS to all the frame sizes
> smaller than 1280,720 ? Or are they filtered out?

However, I agree with Kieran: looking at the code, it sounds to me that
it will indeed duplicate 1/15 and 1/25 intervals.

I suggest add two other intervals there, like:
	12.5 fps and 29.995 fps, e. g.:

static const struct v4l2_fract webcam_intervals[VIVID_WEBCAM_IVALS] = {
        {  1, 1 },
        {  1, 2 },
        {  1, 4 },
        {  1, 5 },
        {  1, 10 },
        {  1, 15 },
	{  2, 50 },
        {  1, 25 },
        {  1, 30 },
        {  1, 40 },
        {  1, 50 },
	{  1001, 30000 },
        {  1, 60 },
};

Provided, of course, that vivid would support producing images
at fractional rate. I didn't check. If not, then simply add
1/20 and 1/40.

> Now the difficulty is adding smaller frame rates (like 1,1, 1,2) would
> effect/reduce the output rates of the larger frame sizes, so how about
> adding some high rate support (any two from 1/{60,75,90,100,120}) instead?

Last week, I got a crash with vivid running at 30 fps, while running an 
event's race code, on a i7core (there, the code was switching all video
controls while subscribing/unsubscribing events). The same code worked
with lower fps.

While I didn't have time to debug it yet, I suspect that it has to do
with the time spent to produce a frame on vivid. So, while it would be
nice to have high rate support, I'm not sure if this is doable. It may,
but perhaps we need to disable some possible video output formats, as some
types may consume more time to build frames.

Thanks,
Mauro