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

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Date:   Mon, 26 Sep 2022 13:17:52 +0200
From:   Thomas Zimmermann <tzimmermann@...e.de>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Maxime Ripard <maxime@...no.tech>
Cc:     Karol Herbst <kherbst@...hat.com>, David Airlie <airlied@...ux.ie>,
        nouveau@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        Phil Elwell <phil@...pberrypi.com>,
        Emma Anholt <emma@...olt.net>,
        Samuel Holland <samuel@...lland.org>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Chen-Yu Tsai <wens@...e.org>, Ben Skeggs <bskeggs@...hat.com>,
        linux-sunxi@...ts.linux.dev, intel-gfx@...ts.freedesktop.org,
        Hans de Goede <hdegoede@...hat.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        linux-arm-kernel@...ts.infradead.org,
        Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
        Dom Cobley <dom@...pberrypi.com>,
        Dave Stevenson <dave.stevenson@...pberrypi.com>,
        linux-kernel@...r.kernel.org,
        Mateusz Kwiatkowski <kfyatek+publicgit@...il.com>,
        Noralf Trønnes <noralf@...nnes.org>
Subject: Re: [PATCH v2 10/33] drm/modes: Add a function to generate analog
 display modes

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@...no.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 ;-)

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@...ux-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

-- 
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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