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

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Message-ID: <CAD=FV=VEMkociw5gyVOtVCPmCyf1b0G7owsAm=o-LxB33YUnjg@mail.gmail.com>
Date: Tue, 24 Sep 2024 09:42:56 -0700
From: Doug Anderson <dianders@...omium.org>
To: Pin-yen Lin <treapking@...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 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...

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)?

-Doug