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

[Thomas Zimmermann]

Hi

Am 26.09.22 um 12:34 schrieb Geert Uytterhoeven:
> Hi Maxime,
>
> On Mon, Sep 26, 2022 at 12:17 PM Maxime Ripard <maxime@xxxxxxxxxx> 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 ;-)

Sort of. It leaves the time value as a magic number, which obfuscates 
what's happening.

The unit for htotal is pixels/scanline because if you multiply it with 
the number of scanlines per frame (which is in vtotal), you get 
pixels/frame. Multiplying with the frames per second results in the 
pixel clock in pixels/second.

That's a bit much for this comment. Hence, I suggested to remove these 
comments entirely and document the relation among the numbers in a more 
prominent location. The documentation for drm_display_mode would be a 
good place, I guess.

Best regards
Thomas

> Gr{oetje,eeting}s,
>
>                          Geert
>
> --
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx
>
> 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

--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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