linux-kernel - Re: [PATCH v2 2/2] media: i2c: ov02e10: add OV02E10 image sensor driver

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Message-ID: <55e070e6-4f01-4db1-bbe8-f8ebc6abd6c1@linaro.org>
Date: Tue, 1 Apr 2025 09:11:19 +0100
From: Bryan O'Donoghue <bryan.odonoghue@...aro.org>
To: Alan Stern <stern@...land.harvard.edu>
Cc: Sakari Ailus <sakari.ailus@...ux.intel.com>,
 Mauro Carvalho Chehab <mchehab@...nel.org>,
 Liam Girdwood <lgirdwood@...il.com>, Mark Brown <broonie@...nel.org>,
 Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>,
 Conor Dooley <conor+dt@...nel.org>, Bryan O'Donoghue <bod@...nel.org>,
 Hans de Goede <hansg@...nel.org>, Jingjing Xiong <jingjing.xiong@...el.com>,
 Hao Yao <hao.yao@...el.com>, Jim Lai <jim.lai@...el.com>,
 You-Sheng Yang <vicamo.yang@...onical.com>,
 Hans de Goede <hdegoede@...hat.com>, linux-kernel@...r.kernel.org,
 linux-media@...r.kernel.org, devicetree@...r.kernel.org
Subject: Re: [PATCH v2 2/2] media: i2c: ov02e10: add OV02E10 image sensor
 driver

On 01/04/2025 03:35, Alan Stern wrote:
> On Tue, Apr 01, 2025 at 01:34:29AM +0100, Bryan O'Donoghue wrote:
>> On 27/03/2025 07:36, Sakari Ailus wrote:
>>>> +static u64 to_pixel_rate(u32 f_index)
>>>> +{
>>>> +	u64 pixel_rate = link_freq_menu_items[f_index] * 2 * OV02E10_DATA_LANES;
>>>> +
>>>> +	do_div(pixel_rate, OV02E10_RGB_DEPTH);
>>> The pixel rate control is for the pixel rate on the pixel array, not on the
>>> CSI-2 interface. Without binning or sub-sampling these may the same still,
>>> but this only works in special cases really.
>>
>> Hmm computer says no, I don't think I have understood this comment..
>>
>> Looking at other drivers, I'd say the above pattern is pretty common -
>> taking ov8856 as an example that's pretty much equivalent logic to the
>> above, ov08x40 does something similar.
>>
>> =>
>>
>> pixel_rate == link_freq * 2 * #oflanes / RGB_DEPTH
>>             => 360MHz * 2 * 2 / 10
>>             => 360000000 * 2 * 2 / 10
>>             => 144000000
>>
>> If I'm understanding you though you mean the pixel rate for the control
>> V4L2_CID_PIXEL_RATE expressed here should be the resolution * the FPS /
>> bits_per_pixel
>>
>> pixel_rate = wdith x height x fps / bpp
>>             => 1928 * 1088 * 30 / 10
>>             => 6292992
>>
>> i.e. the pixel rate not related to the CSI2 link frequency ?
> 
> I know practically nothing about this stuff, but even I can see that this
> suggestion doesn't make sense:
> 
> 	width x height = pixels per frame;
> 	pixels per frame x fps = pixels per second;
> 	pixels per second / bits per pixel = pixels^2 / (bits x seconds)
> 
> which is meaningless.  If the division were replaced with a
> multiplication this would become:
> 
> 	pixels per second x bits per pixel = bits per second
> 
> which at least is reasonable.
> 
> Alan

TBH Perplexity.ai gave me that answer, my question is still are we 
saying the upstream examples are wrong here or have I just not 
understood the original comment ?

---
bod