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Date:   Thu, 18 Aug 2022 14:39:34 +0200
From:   Maxime Ripard <maxime@cerno.tech>
To:     Geert Uytterhoeven <geert@linux-m68k.org>
Cc:     Jernej Skrabec <jernej.skrabec@gmail.com>,
        Martin Blumenstingl <martin.blumenstingl@googlemail.com>,
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Subject: Re: [PATCH v1 04/35] drm/modes: Introduce 480i and 576i modes
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Hi!

On Wed, Aug 17, 2022 at 04:01:48PM +0200, Geert Uytterhoeven wrote:
> > > > > Vertically, it's simpler, as the number of lines is discrete.
> > > > > You do have to take into account interlace and doublescan, and
> > > > > progressive modes with 262/312 lines.
> > > >
> > > > So we only have to deal with 525 and 625 lines total (without taking
> > > > interlace and doublescan into account), right?
> > >
> > > Yes.
> > >
> > > > I guess we still have the same question, we probably want to center=
 it,
> > > > so top =3D=3D bottom, but what about the vsync length?
> > >
> > > Unfortunately that table does not mention top and bottom margins.
> > > But according to drivers/video/fbdev/amifb.c (see the "Broadcast
> > > video timings" comment block and the definitions of the "ntsc-lace"
> > > and "pal-lace" video modes), they are asymmetrical, too.
> > >
> > > Vsync length is 0.576ms, so that's 9 scan lines (I guess I didn't
> > > have that info when I wrote amifb, as I used 4 lines there).
> >
> > Thanks, that's some great info already.
> >
> > It's mentioned though that the settings for NTSC are "straightforward",
> > but it's definitely not for me :)
>=20
> As in NTSC just uses different pixel clock and horizontal/vertical sync
> rate values...

Oh, so the constants differ but the calculation is the same, ack.

> > I've looked around and it looks like the entire blanking area is
> > supposed to be 40 pixels in interlaced, but I couldn't find anywhere how
>=20
> 625 lines - 575[*] visible lines =3D 50 lines.
>=20
> [*] BT.656 uses 576 visible lines as that's a multiple of 2, for splitting
>      a frame in two fields of equal size.
>=20
> "visible" is relative, as it includes the overscan region.
> Some PAL monitors used with computers had knobs to control width/height
> and position of the screen, so you could make use of most or all of
> the overscan region

It brings back some memories :)

> but on a real TV you're limited to ca. 640x512 (on PAL) which is what
> an Amiga used by default (with a 14 MHz pixclock).

> > it's supposed to be split between the upper and lower margins and the
> > sync period.
>=20
> "Field Synchronization of PAL System" on
> http://martin.hinner.info/vga/pal.html shows the split.

Thanks, that's excellent as well.

I'm mostly done with a function that creates a PAL mode, but I still
have one question.

If I understand well, the blanking period is made up (interlace) of 16
pulses for the first field, 14 for the second, each pulse taking half a
line. That amount to 30 pulses, so 15 lines.

I first assumed that the pre-equalizing pulses would be the back porch,
the long sync pulses the vsync, and the post-equalizing pulses the front
porch. But... we're still missing 35 lines to amount to 625 lines, that
seems to be counted in the field itself (305 lines =3D=3D (575 + 35) / 2)

So I guess my assumption was wrong to begin with.

You seem to have used a fixed vsync in amifb to 4 lines, and I don't
understand how you come up with the upper and lower margins (or rather,
how they are linked to what's described in that page)

Maxime

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