Nouveau - Sep 2022 - [PATCH v2 10/33] drm/modes: Add a function to generate analog display modes

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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Hi Maxime,

On Mon, Sep 26, 2022 at 12:17 PM Maxime Ripard <maxime at cerno.tech>
wrote:
> 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?
To make the units add up, it should be 13.5 Mpixel/s
(which is what a pixel clock of 13.5 MHz really means ;-)

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at
linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or
something like that.
                                -- Linus Torvalds