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Date:   Tue, 16 Aug 2022 15:26:36 +0200
From:   Maxime Ripard <maxime@cerno.tech>
To:     Geert Uytterhoeven <geert@linux-m68k.org>
Cc:     Jernej Skrabec <jernej.skrabec@gmail.com>,
        Martin Blumenstingl <martin.blumenstingl@googlemail.com>,
        Chen-Yu Tsai <wens@csie.org>,
        Philipp Zabel <p.zabel@pengutronix.de>,
        Jerome Brunet <jbrunet@baylibre.com>,
        Samuel Holland <samuel@sholland.org>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        Daniel Vetter <daniel@ffwll.ch>, Emma Anholt <emma@anholt.net>,
        David Airlie <airlied@linux.ie>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Noralf =?utf-8?Q?Tr=C3=B8nnes?= <noralf@tronnes.org>,
        Kevin Hilman <khilman@baylibre.com>,
        Neil Armstrong <narmstrong@baylibre.com>,
        linux-sunxi@lists.linux.dev,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Phil Elwell <phil@raspberrypi.com>,
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        Dave Stevenson <dave.stevenson@raspberrypi.com>,
        "open list:ARM/Amlogic Meson..." <linux-amlogic@lists.infradead.org>,
        DRI Development <dri-devel@lists.freedesktop.org>,
        Dom Cobley <dom@raspberrypi.com>
Subject: Re: [PATCH v1 04/35] drm/modes: Introduce 480i and 576i modes
Message-ID: <20220816132636.3tmwqmrox64pu3lt@houat>
References: <20220728-rpi-analog-tv-properties-v1-0-3d53ae722097@cerno.tech>
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Hi Geert,

On Fri, Aug 12, 2022 at 03:18:58PM +0200, Geert Uytterhoeven wrote:
> Hi Maxime,
>=20
> Thanks for your patch!
>=20
> On Fri, Jul 29, 2022 at 6:35 PM Maxime Ripard <maxime@cerno.tech> wrote:
> > Multiple drivers (meson, vc4) define the analog TV 525-lines and 625-li=
nes
> > modes in the drivers.
>=20
> 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.
>=20
> In analog modes, the only discrete values are the number of lines, and
> the frame/field rate (fixing the horizontal sync rate when combined).
>=20
> 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)).
>=20
> 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.

=46rom 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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