linux-kernel - Re: [PATCH 2/2] drm/bridge: it6505: Drop EDID cache on bridge power off

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Message-ID: <CAEXTbpfHUfq6rmSZkXdAYZ-W03t7vMcACUmyco6xL5A4T_Ok=g@mail.gmail.com>
Date: Wed, 25 Sep 2024 12:12:05 +0800
From: Pin-yen Lin <treapking@...omium.org>
To: Doug Anderson <dianders@...omium.org>
Cc: Xin Ji <xji@...logixsemi.com>, Andrzej Hajda <andrzej.hajda@...el.com>, 
	Neil Armstrong <neil.armstrong@...aro.org>, Robert Foss <rfoss@...nel.org>, 
	Laurent Pinchart <Laurent.pinchart@...asonboard.com>, Jonas Karlman <jonas@...boo.se>, 
	Jernej Skrabec <jernej.skrabec@...il.com>, linux-kernel@...r.kernel.org, 
	Sam Ravnborg <sam@...nborg.org>, dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH 2/2] drm/bridge: it6505: Drop EDID cache on bridge power off

Hi,

On Wed, Sep 25, 2024 at 1:48 AM Doug Anderson <dianders@...omium.org> wrote:
>
> Hi,
>
> On Mon, Sep 23, 2024 at 8:53 PM Pin-yen Lin <treapking@...omium.org> wrote:
> >
> > The bridge might miss the display change events when it's powered off.
> > This happens when a user changes the external monitor when the system
> > is suspended and the embedded controller doesn't not wake AP up.
> >
> > It's also observed that one DP-to-HDMI bridge doesn't work correctly
> > when there is no EDID read after it is powered on.
> >
> > Drop the cache to force an EDID read after system resume to fix this.
> >
> > Fixes: 11feaef69d0c ("drm/bridge: it6505: Add caching for EDID")
> > Signed-off-by: Pin-yen Lin <treapking@...omium.org>
>
> Ah, I guess this answers my question in the previous patch about
> whether caching was important even for external displays since this
> driver only supports external DP and the commit you mention in "Fixes"
> says that caching was important.
>
> So this looks reasonable. One thing I wonder is if you're totally
> guaranteed to get a PM Runtime suspend whenever you get an unplug /
> replug of a display. I tried to dig a little bit but I'm not super
> familiar with this bridge and it looks complicated enough that I guess
> I'll have to trust that it's fine. So...

it6505_remove_edid() is also called when the bridge reads HPD low in
the IRQ handler or the DPCD sink count changes to 0 after a HPD_IRQ
signal. The assumption here is that if the bridge is still powered on,
then it should be aware of the monitor change events and drop the EDID
cache when needed. This patch addresses the scenario where monitor
changes are not communicated to the bridge when it is powered off.
>
> Reviewed-by: Douglas Anderson <dianders@...omium.org>

Regards,
Pin-yen