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

[Maxime Ripard]

Hi Geert,

On Fri, Aug 12, 2022 at 03:18:58PM +0200, Geert Uytterhoeven wrote:
> Hi Maxime,
> 
> Thanks for your patch!
> 
> 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?

Thanks
Maxime

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