Hi Maxime,

On Thu, Aug 18, 2022 at 5:46 PM Maxime Ripard [off-list ref] wrote:

On Thu, Aug 18, 2022 at 05:34:30PM +0200, Geert Uytterhoeven wrote:

quoted

On Thu, Aug 18, 2022 at 3:42 PM Maxime Ripard [off-list ref] wrote:

quoted

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@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

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel