linux-kernel - Re: [PATCH v4 3/3] media: mgb4: Fixed signal frame rate limit handling

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Date: Tue, 9 Apr 2024 11:57:02 +0200
From: Martin Tůma <tumic@...see.org>
To: Hans Verkuil <hverkuil@...all.nl>,
 Mauro Carvalho Chehab <mchehab@...nel.org>
Cc: linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
 Martin Tůma <martin.tuma@...iteqautomotive.com>
Subject: Re: [PATCH v4 3/3] media: mgb4: Fixed signal frame rate limit
 handling

On 08. 04. 24 12:47, Hans Verkuil wrote:

>> diff --git a/Documentation/admin-guide/media/mgb4.rst b/Documentation/admin-guide/media/mgb4.rst
>> index 2977f74d7e26..6fff886003e2 100644
>> --- a/Documentation/admin-guide/media/mgb4.rst
>> +++ b/Documentation/admin-guide/media/mgb4.rst
>> @@ -228,8 +228,12 @@ Common FPDL3/GMSL output parameters
>>       open.*
>>   
>>   **frame_rate** (RW):
>> -    Output video frame rate in frames per second. The default frame rate is
>> -    60Hz.
>> +    Output video signal frame rate limit in frames per second. Due to
>> +    the limited output pixel clock steps, the card can not always generate
>> +    a frame rate perfectly matching the value required by the connected display.
>> +    Using this parameter one can limit the frame rate by "crippling" the signal
>> +    so that the lines are not equal but the signal appears like having the exact
>> +    frame rate to the connected display. The default frame rate limit is 60Hz.
> 
> It's not clear what is meant with 'crippling'. Normally when dealing with video
> framerates the driver will pick the closest video timing to the requested framerate.
> It is understood that you can't always get the exact framerate, so drivers can
> make adjustments.
> 

By "crippling" I mean the signal is modified in a obscure way so that a 
frame has not all lines equal. The HW somehow (the exact way is not 
known to me, the documentation is very sparse on this - before my 
investigation it stated only "frames per second" which was very 
confusing and the reason this patch exists) modifies the last 
line(lines?) so that the overall clock ticks per frame is the desired 
value. Some blanking stuff (the porches?) you have set are not equal for 
all the frame lines.

What I'm trying to do is to change the original documentation which is 
definitely wrong (the users are confused how you can set the timings AND 
the frame rate with different values at the same time) without saying 
too much about the exact algorithm as it is not exactly known and may 
even slightly change in different FW versions.

Does that all make sense to you now?