linux-kernel - Re: [PATCH 3/3] drm: sun4i: calculate proper DCLK rate for DSI

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Message-Id: <CRN65FVKWIUG.1VSDAH8INXQMT@iMac.local>
Date:   Mon, 03 Apr 2023 15:52:36 +0200
From:   "Roman Beranek" <me@...y.cz>
To:     "Frank Oltmanns" <frank@...manns.dev>
Cc:     "Maxime Ripard" <mripard@...nel.org>,
        "Chen-Yu Tsai" <wens@...e.org>, "David Airlie" <airlied@...il.com>,
        "Daniel Vetter" <daniel@...ll.ch>,
        "Jernej Skrabec" <jernej.skrabec@...il.com>,
        "Samuel Holland" <samuel@...lland.org>,
        <dri-devel@...ts.freedesktop.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-sunxi@...ts.linux.dev>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] drm: sun4i: calculate proper DCLK rate for DSI

On Sun Apr 2, 2023 at 12:49 PM CEST, Frank Oltmanns wrote:
>
> When apply this to drm-next my panel stays dark. I haven't figured out
> yet why, though. The other two patches in this series work fine, i.e.
> they have no effect as they are just a refactoring.
>
> I'm testing this on my pinephone. It's the same with the patch I
> submitted. For whatever reason, it no longer works on drm-next.

I've reproduced the issue on my PinePhone and noticed that tcon0 had set
pll-video0-2x as its parent instead of pll-mipi. Having tried a whole
range of pll-video0 rates, I'm now convinced that DSI only works when
tcon0 has pll-mipi as its parent.

As little a change as setting .clock in the default mode of PP's panel
to 73500 can fix it. Better yet, dropping pll-video0-2x from the set
of acceptable parents for tcon0 fixes it universally. And that's what
megi's kernel does, though the measure was introduced with a different
rationale:
<https://github.com/megous/linux/commit/7374d5756aa0cc3f11e494e3cbc54f6c7c01e1a8>

Roman