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

[Geert Uytterhoeven]

Hi Maxime,

On Wed, Aug 17, 2022 at 9:54 AM Maxime Ripard <maxime@xxxxxxxxxx> wrote:
> On Tue, Aug 16, 2022 at 05:00:38PM +0200, Geert Uytterhoeven wrote:
> > On Tue, Aug 16, 2022 at 3:26 PM Maxime Ripard <maxime@xxxxxxxxxx> wrote:
> > > On Fri, Aug 12, 2022 at 03:18:58PM +0200, Geert Uytterhoeven wrote:
> > > > On Fri, Jul 29, 2022 at 6:35 PM Maxime Ripard <maxime@xxxxxxxxxx> wrote:
> > > > > Multiple drivers (meson, vc4) define the analog TV 525-lines and 625-lines
> > > > > modes in the drivers.
> > > >
> > > > Nit: strictly speaking these are not analog modes, but the digital
> > > > variants (ITU-R BT.656 and DVD-Video D1) of NTSC and PAL, using a
> > > > 13.5 MHz sampling frequency for pixels.
> > > >
> > > > In analog modes, the only discrete values are the number of lines, and
> > > > the frame/field rate (fixing the horizontal sync rate when combined).
> > > >
> > > > The number of (in)visible pixels per line depends on the available
> > > > bandwidth.  In a digital variant (which is anything generated by a
> > > > digital computer system), the latter depends on the pixel clock, which
> > > > can wildly differ from the 13.5 MHz used in the BT.656 standard. (e.g.
> > > > Amiga uses 7.09/14.19/28.38 MHz (PAL) or 7.16/14.32/28.64 MHz (NTSC)).
> > > >
> > > > So I think we probably need some way to generate a PAL/NTSC-compatible
> > > > mode based not only on resolution, but also on pixel clock.
> > >
> > > This would also fix the comments made by Jani and Thomas, so I quite
> > > like the idea of it.
> > >
> > > I'm struggling a bit to find how would could implement this though.
> > >
> > > From what you were saying, I guess the prototype would be something like
> > >
> > > struct drm_display_mode *drm_create_analog_mode(unsigned int pixel_clock,
> > >                                                 unsigned int lines,
> > >                                                 unsigned int frame_rate)
> > >
> > > But I have zero idea on what the implementation would be. Do you have
> > > some resources for this you could point me to?
> >
> > Horizontally, I think you should calculate left/right margins and
> > hsync length to yield timings that match those for the BT.656 PAL/NTSC
> > modes.  I.e. when a 640x512 mode with a pixel clock of 14 MHz is
> > requested, you want to calculate left', right', and hslen' for
> >
> > | <---- left' ---> | <- 640 pixels -> | <---- right' ---> | <--- hslen' --> |
> >                         @ 14 MHz
> >
> > so they match the timings for left, right, and hslen for
> >
> > | <--- left ---> | <--- 720 pixels ---> | <--- right ---> | <--- hslen ---> |
> >                         @ 13.5 MHz
> >
> > As 640 pixels @ 14 MHz are less wide than 720 pixels @ 13.5 MHz,
> > you want to make sure to align the center of the visible part.
>
> So I guess in that example if we want to center it, left == right and
> left' == right'? What about the sync length?

No, left and right are asymmetrical, cfr. front and back porch in
https://en.wikipedia.org/wiki/PAL#PAL_signal_details
I.e. if the pixel part is reduced, both the left and right margins
should be increased by the same amount.

>From the table linked above, hslen should be ca. 4.7µs (fixed).

> > 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 == 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).

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

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