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

[Geert Uytterhoeven]

Hi Maxime,

On Wed, Aug 17, 2022 at 3:15 PM Maxime Ripard <maxime@xxxxxxxxxx> wrote:
> On Wed, Aug 17, 2022 at 10:51:55AM +0200, Geert Uytterhoeven wrote:
> > 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).
>
> each pixel taking 1 / pixel_clock seconds (assuming pixel_clock is in
> Hz), and thus hslen (in pixels) = 4.7 * 10 ^ -6 * pixel_clk, right?

Exactly.

> > > > 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).
>
> 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 :)

As in NTSC just uses different pixel clock and horizontal/vertical sync
rate values...

> 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

625 lines - 575[*] visible lines = 50 lines.

[*] BT.656 uses 576 visible lines as that's a multiple of 2, for splitting
     a frame in two fields of equal size.

"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, 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.

"Field Synchronization of PAL System" on http://martin.hinner.info/vga/pal.html
shows the split.

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