linux-kernel - Re: [PATCH] drm/sun4i: uncouple DSI dotclock divider from TCON0_DCLK

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Message-Id: <CRUDUEL17ER4.1R8WRNNB5CJ35@void.crly.cz>
Date:   Wed, 12 Apr 2023 03:22:59 +0200
From:   "Roman Beranek" <romanberanek@...oud.com>
To:     Jernej Škrabec <jernej.skrabec@...il.com>,
        "Maxime Ripard" <maxime@...no.tech>
Cc:     "Frank Oltmanns" <frank@...manns.dev>,
        "Chen-Yu Tsai" <wens@...e.org>, "David Airlie" <airlied@...il.com>,
        "Daniel Vetter" <daniel@...ll.ch>,
        "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>,
        <icenowy@...c.io>
Subject: Re: [PATCH] drm/sun4i: uncouple DSI dotclock divider from
 TCON0_DCLK_REG

On Sat Apr 8, 2023 at 9:07 AM CEST, Jernej Škrabec wrote:
> Dne sreda, 05. april 2023 ob 14:34:11 CEST je Roman Beranek napisal(a):
> > While simply forbidding the video0-2x mux option seems
> > to me as the right way to go because there's not much use for it with
> > non-DSI interfaces either besides the opportunity to power pll-mipi
> > down, I'd like to run by you first.
>
> It's been a long time since I looked at A64 HDMI clocks, but IIRC, pll-video0 
> is the only useful source for HDMI PHY (as opposed to HDMI controller.)
> So question remains how to properly support both displays at the same time.
>

Correct.

> Have you ever tried to make HDMI and DSI work at the same time? This is one of 
> issues of the PinePhone IIUC.
>

Yes, I have. Prusa3D's SL1 printer, on which I previously worked on,
uses both outputs simultaneously. I had encountered the same reparenting
problem back then but since I hadn't been able to identify it,
I resorted to fiddling with the DSI pixelclock until it worked. 

DSI & HDMI co-existence is yet another reasoni though for forbidding
the pll-video-2x parent. megi's kernel includes Mr. Zheng's commit which
does the same.

<https://github.com/megous/linux/commit/7374d57>

Best wishes
Roman Beranek