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Date:   Mon, 26 Sep 2022 12:17:16 +0200
From:   Maxime Ripard <maxime@cerno.tech>
To:     Thomas Zimmermann <tzimmermann@suse.de>
Cc:     Jernej Skrabec <jernej.skrabec@gmail.com>,
        Rodrigo Vivi <rodrigo.vivi@intel.com>,
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Subject: Re: [PATCH v2 10/33] drm/modes: Add a function to generate analog
 display modes
Message-ID: <20220926101716.urehomr2lzv5pqln@houat>
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Hi,

On Fri, Sep 23, 2022 at 11:05:48AM +0200, Thomas Zimmermann wrote:
> > +	/* 63.556us * 13.5MHz =3D 858 pixels */
>=20
> 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 (=3D=3D pixel) is ~74ns, so we get 63556 / 74 =3D 858
pixels / line.

> I think you want to end up with 858 pixels/line =3D
>=20
>   13,5 pixels/second / (60/2I frame/second * 525 lines/frame)
>=20
> I: interlaced
>=20
> Maybe just remove the short comments and document that in a more meaningf=
ul
> 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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