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

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Date:   Tue, 16 Aug 2022 15:26:36 +0200
From:   Maxime Ripard <maxime@...no.tech>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Jernej Skrabec <jernej.skrabec@...il.com>,
        Martin Blumenstingl <martin.blumenstingl@...glemail.com>,
        Chen-Yu Tsai <wens@...e.org>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Jerome Brunet <jbrunet@...libre.com>,
        Samuel Holland <samuel@...lland.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Daniel Vetter <daniel@...ll.ch>, Emma Anholt <emma@...olt.net>,
        David Airlie <airlied@...ux.ie>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Noralf Trønnes <noralf@...nnes.org>,
        Kevin Hilman <khilman@...libre.com>,
        Neil Armstrong <narmstrong@...libre.com>,
        linux-sunxi@...ts.linux.dev,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Phil Elwell <phil@...pberrypi.com>,
        Mateusz Kwiatkowski <kfyatek+publicgit@...il.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Dave Stevenson <dave.stevenson@...pberrypi.com>,
        "open list:ARM/Amlogic Meson..." <linux-amlogic@...ts.infradead.org>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        Dom Cobley <dom@...pberrypi.com>
Subject: Re: [PATCH v1 04/35] drm/modes: Introduce 480i and 576i modes

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@...no.tech> 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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