[PATCH v1 04/35] drm/modes: Introduce 480i and 576i modes

[Geert Uytterhoeven]

Hi Maxime,

On Thu, Aug 18, 2022 at 5:46 PM Maxime Ripard <maxime at cerno.tech> wrote:
> On Thu, Aug 18, 2022 at 05:34:30PM +0200, Geert Uytterhoeven wrote:
> > On Thu, Aug 18, 2022 at 3:42 PM Maxime Ripard <maxime at cerno.tech> wrote:
> > > I started adding more sanity checks to my code, and I just realised I
> > > don't seem to be able to reach 720 pixels over a single line though. If
> > > I understood it properly, and according to [1] the active part of a line
> > > is supposed to be 51.95us, and the blanking period taking 12.05us. [2]
> > > in the timing section has pretty much the same numbers, so it looks
> > > sane.
> > >
> > > At 13.5Mhz, a pixel is going to take roughly 74ns, and 51950 / 74 = 702
> > > pixels
> > >
> > > It seems we can go push it to 52350 ns, but that still gives us only 706
> > > pixels.
> > >
> > > Similarly, if I just choose to ignore that limit and just take the
> > > active time I need, 720 * 74 = 53280ns
> > >
> > > That leaves us 10720ns for the blanking period, and that's not enough to
> > > fit even the minimum of the front porch, hsync and back porch (1.55 +
> > > 4.5 + 5.5 = 11.55us).
> > >
> > > Are those constraints merely recommendations, or am I missing something?
> >
> > You are missing that the parts near the borders of the full image are
> > part of the overscan range, and may or may not be visible, depending
> > on your actual display.
> > The full 768x576 image size from BT.656 is not visible on a typical PAL display,
> > and is more of an "absolute maximum rating", guaranteed to cover more
> > than analog PAL.
>
> So the overscan range is not part of the active area, unlike what HDMI
> is doing for example?

Indeed. DVI-D and HDMI etc. are pure digital (let's ignore they are a
digitized variant of old analog VGA ;-), hence there is a one-to-one
match between pixels in the image and pixels on the screen (ignoring
scaling).  But even when using an analog VGA input on a modern
digital display, you have controls to e.g. move the image.

> Is there some minimal timings available somewhere to fit those absolute
> maximum ratings?

I guess they can be found on the Internet...

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-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