linux-kernel - Re: [PATCH] drm: Document that power requirements for DP AUX transfers

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Message-ID: <YnJv3B/85hTz54SC@intel.com>
Date:   Wed, 4 May 2022 15:21:48 +0300
From:   Ville Syrjälä <ville.syrjala@...ux.intel.com>
To:     Douglas Anderson <dianders@...omium.org>
Cc:     dri-devel@...ts.freedesktop.org, Lyude Paul <lyude@...hat.com>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Maxime Ripard <maxime@...no.tech>,
        linux-arm-msm@...r.kernel.org, freedreno@...ts.freedesktop.org,
        Stephen Boyd <swboyd@...omium.org>,
        Robert Foss <robert.foss@...aro.org>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Hsin-Yi Wang <hsinyi@...omium.org>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...ux.ie>,
        Imre Deak <imre.deak@...el.com>,
        Jani Nikula <jani.nikula@...el.com>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm: Document that power requirements for DP AUX
 transfers

On Tue, May 03, 2022 at 04:21:08PM -0700, Douglas Anderson wrote:
> When doing DP AUX transfers there are two actors that need to be
> powered in order for the DP AUX transfer to work: the DP source and
> the DP sync. Commit bacbab58f09d ("drm: Mention the power state
> requirement on side-channel operations") added some documentation
> saying that the DP source is required to power itself up (if needed)
> to do AUX transfers. However, that commit doesn't talk anything about
> the DP sink.
> 
> For full fledged DP the sink isn't really a problem. It's expected
> that if an external DP monitor isn't plugged in that attempting to do
> AUX transfers won't work. It's also expected that if a DP monitor is
> plugged in (and thus asserting HPD) that it AUX transfers will work.
> 
> When we're looking at eDP, however, things are less obvious. Let's add
> some documentation about expectations. Here's what we'll say:
> 
> 1. We don't expect the DP AUX transfer function to power on an eDP
> panel. If an eDP panel is physically connected but powered off then it
> makes sense for the transfer to fail.

I don't agree with this. I think the panel should just get powred up
for AUX transfers. Otherwise you can't trust that eg. the /dev/aux
stuff is actually usable.

-- 
Ville Syrjälä
Intel