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 14:42:59 +0200
From:   Maxime Ripard <maxime@...no.tech>
To:     Thomas Zimmermann <tzimmermann@...e.de>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        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

On Mon, Sep 26, 2022 at 01:17:52PM +0200, Thomas Zimmermann wrote:
> 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 true, but both are true?

> 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.

I'm not sure I understand what it's about. It's an explicit requirement
of PAL and NTSC, why would something so specific be in the generic
definition of drm_display_mode?

Maxime

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