Re: [PATCH v2 10/33] drm/modes: Add a function to generate analog display modes

[Maxime Ripard]

Hi,

On Fri, Sep 23, 2022 at 11:05:48AM +0200, Thomas Zimmermann wrote:
> > +	/* 63.556us * 13.5MHz = 858 pixels */
> 
> I kind of get what the comment wants to tell me, but the units don't add up.

I'm not sure how it doesn't add up?

We have a frequency in Hz (equivalent to s^-1) and a duration in s, so
the result ends up with no dimension, which is to be expected for a
number of periods?

If you're talking about the comment itself, then NTSC mandates that a
line is 63.556us long. If we're using a pixel clock at 13.5 MHz, it
means that the period (== pixel) is ~74ns, so we get 63556 / 74 = 858
pixels / line.

> I think you want to end up with 858 pixels/line =
> 
>   13,5 pixels/second / (60/2I frame/second * 525 lines/frame)
> 
> I: interlaced
> 
> Maybe just remove the short comments and document that in a more meaningful
> place.

I guess this is where it's meaningful, we really want to hit that
target. BT601 also mandates it.

Maxime

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