linux-kernel - Re: [PATCH 0/3] Handling of reduced FPS in V4L2

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Date:   Fri, 24 Mar 2017 13:28:08 +0100
From:   Hans Verkuil <hansverk@...co.com>
To:     Jose Abreu <Jose.Abreu@...opsys.com>, linux-media@...r.kernel.org
Cc:     Carlos Palminha <CARLOS.PALMINHA@...opsys.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Hans Verkuil <hans.verkuil@...co.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3] Handling of reduced FPS in V4L2

On 03/24/17 13:21, Jose Abreu wrote:
> Hi Hans,
> 
> 
> On 24-03-2017 12:12, Hans Verkuil wrote:
>> On 03/24/17 12:52, Jose Abreu wrote:
>>> Hi Hans,
>>>
>>>
>>>>> Can you please review this series, when possible? And if you
>>>>> could test it on cobalt it would be great :)
>>>> Hopefully next week. 
>>> Thanks :)
>>>
>>>> Did you have some real-world numbers w.r.t. measured
>>>> pixelclock frequencies and 60 vs 59.94 Hz and 24 vs 23.976 Hz?
>>> I did make some measurements but I'm afraid I didn't yet test
>>> with many sources (I mostly tested with signal generators which
>>> should have a higher precision clock than real sources). I have a
>>> bunch of players here, I will test them as soon as I can.
>>> Regarding precision: for our controller is theoretically and
>>> effectively enough: The worst case is for 640x480, and even in
>>> that case the difference between 60Hz and 59.94Hz is > 1 unit of
>>> the measuring register. This still doesn't solve the problem of
>>> having a bad source with a bad clock, but I don't know if we can
>>> do much more about that.
>> I would really like to see a table with different sources sending
>> these different framerates and the value that your HW detects.
>>
>> If there is an obvious and clear difference, then this feature makes
>> sense. If it is all over the place, then I need to think about this
>> some more.
>>
>> To be honest, I expect that you will see 'an obvious and clear'
>> difference, but that is no more than a gut feeling at the moment and
>> I would like to see some proper test results.
> 
> Ok, I will make a table. The test procedure will be like this:
>     - Measure pixel clock value using certified HDMI analyzer
>     - Measure pixel clock using our controller
>     - Compare the values obtained from analyzer, controller and
> the values that the source is telling to send (the value
> displayed in source menu for example [though, some of them may
> not discriminate the exact frame rate, thats why analyzer should
> be used also]).
> 
> Seems ok? I will need some time, something like a week because my
> setup was "borrowed".

That sounds good. Sorry for adding to your workload, but there is no
point to have a flag that in practice is meaningless.

I'm actually very curious about the results!

Regards,

	Hans

> 
> Best regards,
> Jose Miguel Abreu
> 
>>
>>>> I do want to see that, since this patch series only makes sense if you can
>>>> actually make use of it to reliably detect the difference.
>>>>
>>>> I will try to test that myself with cobalt, but almost certainly I won't
>>>> be able to tell the difference; if memory serves it can't detect the freq
>>>> with high enough precision.
>>> Ok, thanks, this would be great because I didn't test the series
>>> exactly "as is" because I'm using 4.10. I did look at vivid
>>> driver but it already handles reduced frame rate, so it kind of
>>> does what it is proposed in this series. If this helper is
>>> integrated in the v4l2 core then I can send the patch to vivid.
>> That would be nice to have in vivid.
>>
>> Regards,
>>
>> 	Hans
>>
>