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From: Doug Anderson <dianders@chromium.org>
Date: Wed, 28 Feb 2024 08:40:22 -0800
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Subject: Re: [PATCH] drm/dp: Don't attempt AUX transfers when eDP panels are
 not powered
To: neil.armstrong@linaro.org
Cc: Jani Nikula <jani.nikula@intel.com>, Maxime Ripard <mripard@kernel.org>, 
	Thomas Zimmermann <tzimmermann@suse.de>, Maarten Lankhorst <maarten.lankhorst@linux.intel.com>, 
	Hsin-Yi Wang <hsinyi@chromium.org>, dri-devel@lists.freedesktop.org, eizan@chromium.org, 
	Ankit Nautiyal <ankit.k.nautiyal@intel.com>, Daniel Vetter <daniel@ffwll.ch>, 
	David Airlie <airlied@gmail.com>, Heiner Kallweit <hkallweit1@gmail.com>, 
	Imre Deak <imre.deak@intel.com>, Jessica Zhang <quic_jesszhan@quicinc.com>, 
	Sam Ravnborg <sam@ravnborg.org>, Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>, 
	linux-kernel@vger.kernel.org
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Neil,

On Thu, Feb 15, 2024 at 8:53=E2=80=AFAM Neil Armstrong
<neil.armstrong@linaro.org> wrote:
>
> Hi Doug,
>
> On 15/02/2024 16:08, Doug Anderson wrote:
> > Hi,
> >
> > On Thu, Feb 15, 2024 at 2:24=E2=80=AFAM Jani Nikula <jani.nikula@intel.=
com> wrote:
> >>
> >> On Wed, 14 Feb 2024, Doug Anderson <dianders@chromium.org> wrote:
> >>> Hi,
> >>>
> >>> On Tue, Feb 13, 2024 at 10:25=E2=80=AFPM Hsin-Yi Wang <hsinyi@chromiu=
m.org> wrote:
> >>>>
> >>>> On Wed, Feb 14, 2024 at 2:23=E2=80=AFPM Douglas Anderson <dianders@c=
hromium.org> wrote:
> >>>>>
> >>>>> If an eDP panel is not powered on then any attempts to talk to it o=
ver
> >>>>> the DP AUX channel will timeout. Unfortunately these attempts may b=
e
> >>>>> quite slow. Userspace can initiate these attempts either via a
> >>>>> /dev/drm_dp_auxN device or via the created i2c device.
> >>>>>
> >>>>> Making the DP AUX drivers timeout faster is a difficult proposition=
.
> >>>>> In theory we could just poll the panel's HPD line in the AUX transf=
er
> >>>>> function and immediately return an error there. However, this is
> >>>>> easier said than done. For one thing, there's no hard requirement t=
o
> >>>>> hook the HPD line up for eDP panels and it's OK to just delay a fix=
ed
> >>>>> amount. For another thing, the HPD line may not be fast to probe. O=
n
> >>>>> parade-ps8640 we need to wait for the bridge chip's firmware to boo=
t
> >>>>> before we can get the HPD line and this is a slow process.
> >>>>>
> >>>>> The fact that the transfers are taking so long to timeout is causin=
g
> >>>>> real problems. The open source fwupd daemon sometimes scans DP buss=
es
> >>>>> looking for devices whose firmware need updating. If it happens to
> >>>>> scan while a panel is turned off this scan can take a long time. Th=
e
> >>>>> fwupd daemon could try to be smarter and only scan when eDP panels =
are
> >>>>> turned on, but we can also improve the behavior in the kernel.
> >>>>>
> >>>>> Let's let eDP panels drivers specify that a panel is turned off and
> >>>>> then modify the common AUX transfer code not to attempt a transfer =
in
> >>>>> this case.
> >>>>>
> >>>>> Signed-off-by: Douglas Anderson <dianders@chromium.org>
> >>>>> ---
> >>>>
> >>>> Reviewed-by: Hsin-Yi Wang <hsinyi@chromium.org>
> >>>
> >>> Thanks for the review!
> >>>
> >>> Given that this touches core DRM code and that I never got
> >>> confirmation that Jani's concerns were addressed with my previous
> >>> response, I'm still going to wait a little while before applying. I'm
> >>> on vacation for most of next week, but if there are no further replie=
s
> >>> between now and then I'll plan to apply this to "drm-misc-next" the
> >>> week of Feb 26th. If someone else wants to apply this before I do the=
n
> >>> I certainly won't object. Jani: if you feel this needs more discussio=
n
> >>> or otherwise object to this patch landing then please yell. Likewise
> >>> if anyone else in the community wants to throw in their opinion, feel
> >>> free.
> >>
> >> Sorry for dropping the ball after my initial response. I simply have n=
ot
> >> had the time to look into this.
> >>
> >> It would be great to get, say, drm-misc maintainer ack on this before
> >> merging. It's not fair for me to stall this any longer, I'll trust the=
ir
> >> judgement.
> >>
> >> Reasonable?
> >
> > I'd be more than happy for one of the drm-misc maintainers to Ack.
> > I'll move Maxime, Thomas, and Maarten to the "To:" line to see if that
> > helps get through their filters.
>
> I'll like some test reports to be sure it doesn't break anything,
> then I'll be happy to give my ack !

Are you looking for any more test reports at this point? Eizan did
some testing and provided a tag, though this was also on ChromeOS.
Steev also tested on two non-ChromeOS environments and provided his
tag. It's also been another two weeks of this being rolled out to some
Chromebook users and I haven't heard any reports of problems. If
somehow something was missed, I'm happy to follow-up and provide
additional fixes if some report comes in later.

Thanks!

-Doug