linux-kernel - Re: [PATCH v2 10/41] drm/modes: Add a function to generate analog display modes

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Message-ID: <411ebe0f-f398-8e32-ad0e-b42732aa880f@gmail.com>
Date:   Sun, 11 Sep 2022 06:30:39 +0200
From:   kFYatek <kfyatek@...il.com>
To:     Maxime Ripard <maxime@...no.tech>
Cc:     Ben Skeggs <bskeggs@...hat.com>, David Airlie <airlied@...ux.ie>,
        Chen-Yu Tsai <wens@...e.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Lyude Paul <lyude@...hat.com>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Samuel Holland <samuel@...lland.org>,
        Karol Herbst <kherbst@...hat.com>,
        Noralf Trønnes <noralf@...nnes.org>,
        Emma Anholt <emma@...olt.net>, Daniel Vetter <daniel@...ll.ch>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Hans de Goede <hdegoede@...hat.com>,
        linux-arm-kernel@...ts.infradead.org,
        Phil Elwell <phil@...pberrypi.com>,
        intel-gfx@...ts.freedesktop.org,
        Dave Stevenson <dave.stevenson@...pberrypi.com>,
        dri-devel@...ts.freedesktop.org, Dom Cobley <dom@...pberrypi.com>,
        linux-kernel@...r.kernel.org, nouveau@...ts.freedesktop.org,
        linux-sunxi@...ts.linux.dev,
        Geert Uytterhoeven <geert@...ux-m68k.org>
Subject: Re: [PATCH v2 10/41] drm/modes: Add a function to generate analog
 display modes

Hi Maxime,

W dniu 9.09.2022 o 16:00, Maxime Ripard pisze:
> On Wed, Sep 07, 2022 at 11:31:21PM +0200, Mateusz Kwiatkowski wrote:
>> The "canonical" modelines (at least for vc4's VEC, see the notes below):
>>
>> - (vfp==4, vsync==6, vbp==39) for 576i
>> - (vfp==7, vsync==6, vbp==32) for 480i
>> - (vfp==5, vsync==6, vbp==28) for 486i (full frame NTSC as originally specified)
>
> It's not clear to me either how you come up with those timings?

Well, experimentally ;)

The values for 480i and 576i are the values currently used by the downstream
kernel, and those in turn has been copied from the firmware (or more precisely,
I chose them so that the PV registers end up the same as the values set by the
firmware).

I also checked with an oscilloscope that the waveforms look as they should.
VEC doesn't exactly handle the half-lines at the start and end of the odd field
right, but otherwise, the blanking and sync pulses are where they should be.

The 486i values has been constructed from the 480i ones according to the
calculations from cross-referencing SMPTE documents, see my previous e-mail.

I know this is perhaps unsatisfactory ;) I don't have access to the VC4
documentation, so this was _almost_ reverse engineering for me.

Best regards,
Mateusz Kwiatkowski