linux-kernel - Re: [PATCH 1/2] drm/bridge: anx7625: Drop EDID cache on bridge power off

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Message-ID: <CAEXTbpe4R8rXHn1+e55RxzryNW-0YexQfr7M5CBz2hP9qneTQA@mail.gmail.com>
Date: Wed, 25 Sep 2024 12:01:09 +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 1/2] drm/bridge: anx7625: Drop EDID cache on bridge power off

Hi,

On Wed, Sep 25, 2024 at 12:43 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: 8bdfc5dae4e3 ("drm/bridge: anx7625: Add anx7625 MIPI DSI/DPI to DP")
> > Signed-off-by: Pin-yen Lin <treapking@...omium.org>
> > ---
> >
> >  drivers/gpu/drm/bridge/analogix/anx7625.c | 1 +
> >  1 file changed, 1 insertion(+)
>
> I'm not totally sure if it matters, but I wonder if you should change
> this to only delete the EDID cache if you're in DP mode and not eDP
> mode? For eDP mode the panel is not allowed to change and re-reading
> it needlessly seems like it would slow down things like
> suspend/resume. I think this would only matter if someone were using
> eDP panels in the "old" way (not under the aux-bus) because we don't
> set the "DRM_BRIDGE_OP_EDID" when we see "aux-bus", so maybe we don't
> care that much but still...

I'll update this in v2.
>
> Other than that, I know that there have been discussions in the past
> about EDID caches but I can't quite remember all the details. I know
> that panel-edp.c still caches it, so we must have concluded that it's
> at least fine/reasonable for panels. I don't remember whether caching
> is encouraged / suggested for external displays, though. Do you happen
> to know if it even makes a difference there (in other words, do you
> actually see multiple calls to read the EDID when you plug in a DP
> display)?

At least on ChromeOS, Chrome triggers two EDID reads when I plug in an
external monitor. I'm not sure if this is common for other DRM masters
though.
>
> -Doug

Regards,
Pin-yen