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

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Message-ID: <9c6cb27d-dd9c-4cb5-bd84-5b11ae099778@rowland.harvard.edu>
Date: Mon, 31 Mar 2025 22:35:47 -0400
From: Alan Stern <stern@...land.harvard.edu>
To: Bryan O'Donoghue <bryan.odonoghue@...aro.org>
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 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