Date:   Mon, 13 Mar 2017 19:03:14 +0000
From:   Jose Abreu <Jose.Abreu@...opsys.com>
To:     Hans Verkuil <hverkuil@...all.nl>,
        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>,
        Charles-Antoine Couret <charles-antoine.couret@...vision.fr>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] [media] v4l2-dv-timings: Introduce v4l2_calc_fps()

Hi Hans,


On 09-03-2017 15:40, Hans Verkuil wrote:
> On 09/03/17 16:15, Jose Abreu wrote:
>> Hi Hans,
>>
>>
>> Thanks for the review!
>>
>>
>> On 09-03-2017 12:29, Hans Verkuil wrote:
>>> On 07/03/17 17:48, Jose Abreu wrote:
>>>> HDMI Receivers receive video modes which, according to
>>>> CEA specification, can have different frames per second
>>>> (fps) values.
>>>>
>>>> This patch introduces a helper function in the media core
>>>> which can calculate the expected video mode fps given the
>>>> pixel clock value and the horizontal/vertical values. HDMI
>>>> video receiver drivers are expected to use this helper so
>>>> that they can correctly fill the v4l2_streamparm structure
>>>> which is requested by vidioc_g_parm callback.
>>>>
>>>> We could also use a lookup table for this but it wouldn't
>>>> correctly handle 60Hz vs 59.94Hz situations as this all
>>>> depends on the pixel clock value.
>>>>
>>>> Signed-off-by: Jose Abreu <joabreu@...opsys.com>
>>>> Cc: Carlos Palminha <palminha@...opsys.com>
>>>> Cc: Mauro Carvalho Chehab <mchehab@...nel.org>
>>>> Cc: Hans Verkuil <hans.verkuil@...co.com>
>>>> Cc: Charles-Antoine Couret <charles-antoine.couret@...vision.fr>
>>>> Cc: linux-media@...r.kernel.org
>>>> Cc: linux-kernel@...r.kernel.org
>>>> ---
>>>>  drivers/media/v4l2-core/v4l2-dv-timings.c | 29 +++++++++++++++++++++++++++++
>>>>  include/media/v4l2-dv-timings.h           |  8 ++++++++
>>>>  2 files changed, 37 insertions(+)
>>>>
>>>> diff --git a/drivers/media/v4l2-core/v4l2-dv-timings.c b/drivers/media/v4l2-core/v4l2-dv-timings.c
>>>> index 5c8c49d..19946c6 100644
>>>> --- a/drivers/media/v4l2-core/v4l2-dv-timings.c
>>>> +++ b/drivers/media/v4l2-core/v4l2-dv-timings.c
>>>> @@ -814,3 +814,32 @@ struct v4l2_fract v4l2_calc_aspect_ratio(u8 hor_landscape, u8 vert_portrait)
>>>>  	return aspect;
>>>>  }
>>>>  EXPORT_SYMBOL_GPL(v4l2_calc_aspect_ratio);
>>>> +
>>>> +struct v4l2_fract v4l2_calc_fps(const struct v4l2_dv_timings *t)
>>>> +{
>>>> +	const struct v4l2_bt_timings *bt = &t->bt;
>>>> +	struct v4l2_fract fps_fract = { 1, 1 };
>>>> +	unsigned long n, d;
>>>> +	unsigned long mask = GENMASK(BITS_PER_LONG - 1, 0);
>>> This is wrong since v4l2_fract uses u32, and LONG can be 64 bits.
>> Yes, its wrong. I will remove the variable and just use fps, 100
>> instead of mask, mask.
>>
>>>> +	u32 htot, vtot, fps;
>>>> +	u64 pclk;
>>>> +
>>>> +	if (t->type != V4L2_DV_BT_656_1120)
>>>> +		return fps_fract;
>>>> +
>>>> +	htot = V4L2_DV_BT_FRAME_WIDTH(bt);
>>>> +	vtot = V4L2_DV_BT_FRAME_HEIGHT(bt);
>>>> +	pclk = bt->pixelclock;
>>>> +	if (bt->interlaced)
>>>> +		htot /= 2;
>>> This can be dropped. This is the timeperframe, not timeperfield. So for interleaved
>>> formats the time is that of two fields (aka one frame).
>> Ok, but then there is something not correct in
>> v4l2_dv_timings_presets structure field values because I get
>> wrong results in double clocked modes. I checked the definition
>> and the modes that are double clocked are defined with half the
>> clock, i.e., V4L2_DV_BT_CEA_720X480I59_94 is defined with a pixel
>> clock of 13.5MHz but in CEA spec this mode is defined with pixel
>> clock of 27MHz.
> It's defined in the CEA spec as 1440x480 which is the double clocked
> version of 720x480.
>
> The presets are defined without any pixel repeating. In fact, no driver
> that is in the kernel today supports pixel repeating. Mostly because there was
> never any need since almost nobody uses resolutions that require this.
>
> If you decide to add support for this, then it would not surprise me if
> some of the core dv-timings support needs to be adjusted.
>
> To be honest, I never spent time digging into the pixel repeating details,
> so I am not an expert on this at all.
>
>>>> +
>>>> +	fps = (htot * vtot) > 0 ? div_u64((100 * pclk), (htot * vtot)) : 0;
>>>> +
>>>> +	rational_best_approximation(fps, 100, mask, mask, &n, &d);
>>> I think you can just use fps, 100 instead of mask, mask.
>>>
>>> What is returned if fps == 0?
>> I will add a check for this.
>>
>>> I don't have a problem as such with this function, but just be aware that the
>>> pixelclock is never precise: there are HDMI receivers that are unable to report
>>> the pixelclock with enough precision to even detect if it is 60 vs 59.94 Hz.
>>>
>>> And even for those that can, it is often not reliable.
>> My initial intention for this function was that it should be used
>> with v4l2_find_dv_timings_cea861_vic, when possible. That is,
>> HDMI receivers have access to AVI infoframe contents. Then they
>> should get the vic, call v4l2_find_dv_timings_cea861_vic, get
>> timings and then call v4l2_calc_fps to get fps. If no AVI
>> infoframe is available then it should resort to pixel clock and
>> H/V measures as last resort.
> Right, but there are no separate VIC codes for 60 vs 59.94 Hz. Any vertical
> refresh rate that can be divided by 6 can also support these slightly lower
> refresh rates. The timings returned by v4l2_find_dv_timings_cea861_vic just
> report if that is possible, but the pixelclock is set for 24, 30 or 60 fps.

Right, I was forgetting about this ...

So:
1) Most of HDMI receivers do not have the expected precision in
measuring pixel clock value;
2) Most (I would guess all of them?) have access to AVI infoframe
contents;
3) The FPS value is generally used by applications to calculate
expected frame rate and number of frames dropped (right?);
4) The factor in FPS value can be adjusted by 1000/1001;

>From these points I would propose in just using the vic and drop
the resolution in fps a little bit, do you agree?

>
> Perhaps I should see the driver code...

Can't publish it yet :/


Best regards,
Jose Miguel Abreu

>
>>> In order for me to merge this it also should be used in a driver. Actually the
>>> cobalt and vivid drivers would be suitable: you can test the vivid driver yourself,
>>> and if you have a patch for the cobalt driver, then I can test that for you.
>>>
>>> Would be nice for the cobalt driver, since g_parm always returns 60 fps :-)
>> Ok, I will check what I can do :)
>>
>> Best regards,
>> Jose Miguel Abreu
> Regards,
>
> 	Hans
>

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