linux-kernel - Re: [PATCH v1 20/35] drm/vc4: vec: Switch for common modes

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Date:   Tue, 16 Aug 2022 13:57:06 +0200
From:   Maxime Ripard <maxime@...no.tech>
To:     Mateusz Kwiatkowski <kfyatek@...il.com>
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@...r.kernel.org,
        Phil Elwell <phil@...pberrypi.com>,
        linux-arm-kernel@...ts.infradead.org,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Dave Stevenson <dave.stevenson@...pberrypi.com>,
        linux-amlogic@...ts.infradead.org, dri-devel@...ts.freedesktop.org,
        Dom Cobley <dom@...pberrypi.com>
Subject: Re: [PATCH v1 20/35] drm/vc4: vec: Switch for common modes

Hi,

On Fri, Jul 29, 2022 at 08:12:07PM +0200, Mateusz Kwiatkowski wrote:
> I'm just noting that the modelines you defined in drm_modes.c are different
> to the ones you're removing here.
> 
> The horizontal sync differences probably doesn't matter too much, VC4 uses
> those only as a hint anyway and generates sync autonomously, so the slight
> differences will only cause the image to slightly shift horizontally.
> 
> But you're also changing the 480i vertical sync (vsync_start is now 488
> instead of 487, etc.). Are you sure that this won't break anything? This
> will probably shift the image by 1 line (which for the 480i might actually
> mean going out of spec), and I _think_ it might control the odd vs. even
> field first modes on some drivers. I won't be able to test this before
> Monday, but I'm just pointing out the potential issue.

I didn't see any difference on both vc4 and sun4i, but you might be
right about this.

I didn't have much confidence in the vc4 modes since they were broken
before your patches, but maybe I should have used yours still.

> BTW, I've seen a similar thing in the sun4i driver changes (patch 32/35) and
> the differences in vertical sync are even more dramatic. It's entirely
> possible that the current timings in sun4i are broken and the new ones are
> correct (the new ones certainly look saner to me), but I'd double-check if
> that driver does not have any quirks that would _require_ such weird
> settings.

The only thing sun4i requires from the new mode is the line number
anyway, which stays the same.

Maxime

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