linux-kernel - Re: [PATCH v3 3/8] drm/msm/dp: Read DPCD and sink count in bridge detect()

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Message-ID: <35swyvpxv6xf64lgl2o6hy363ahjyv6gcrfnv76vcgec5qxmb2@etujn65vcwho>
Date: Thu, 15 Jan 2026 20:48:23 +0200
From: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
To: Bjorn Andersson <andersson@...nel.org>
Cc: Rob Clark <robin.clark@....qualcomm.com>,
        Dmitry Baryshkov <lumag@...nel.org>,
        Abhinav Kumar <abhinav.kumar@...ux.dev>,
        Jessica Zhang <jesszhan0024@...il.com>, Sean Paul <sean@...rly.run>,
        Marijn Suijten <marijn.suijten@...ainline.org>,
        David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
        Kuogee Hsieh <quic_khsieh@...cinc.com>, linux-arm-msm@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, freedreno@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org,
        Jessica Zhang <jessica.zhang@....qualcomm.com>
Subject: Re: [PATCH v3 3/8] drm/msm/dp: Read DPCD and sink count in bridge
 detect()

On Thu, Jan 15, 2026 at 08:57:24AM -0600, Bjorn Andersson wrote:
> On Thu, Jan 15, 2026 at 09:29:08AM +0200, Dmitry Baryshkov wrote:
> > From: Jessica Zhang <jessica.zhang@....qualcomm.com>
> > 
> > Instead of relying on the link_ready flag to specify if DP is connected,
> > read the DPCD bits and get the sink count to accurately detect if DP is
> > connected.
> 
> This makes it sounds like the two options are equal, but they most
> definitely aren't.
> 
> I think this commit message should capture the fact that "link_ready"
> not only says that the cable is connected, but that we've managed to
> bring up the main link - which is a source of race conditions in the hot
> plug detection logic, as well as making it impossible to move link
> management to the enable/disable calls.
> 
> > 
> > Signed-off-by: Jessica Zhang <jessica.zhang@....qualcomm.com>
> > Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
> > ---
> >  drivers/gpu/drm/msm/dp/dp_display.c | 60 +++++++++++++++++++++++++++++++++++++
> >  drivers/gpu/drm/msm/dp/dp_drm.c     | 20 -------------
> >  drivers/gpu/drm/msm/dp/dp_drm.h     |  2 ++
> >  3 files changed, 62 insertions(+), 20 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/msm/dp/dp_display.c b/drivers/gpu/drm/msm/dp/dp_display.c
> > index 5997cd28ba11..a05144de3b93 100644
> > --- a/drivers/gpu/drm/msm/dp/dp_display.c
> > +++ b/drivers/gpu/drm/msm/dp/dp_display.c
> > @@ -1151,6 +1151,66 @@ static int msm_dp_hpd_event_thread_start(struct msm_dp_display_private *msm_dp_p
> >  	return 0;
> >  }
> >  
> > +/**
> > + * msm_dp_bridge_detect - callback to determine if connector is connected
> > + * @bridge: Pointer to drm bridge structure
> > + * @connector: Pointer to drm connector structure
> > + * Returns: Bridge's 'is connected' status
> 
> Could you please rewrite the return definition, to capture what the
> value really refers to.
> 
> > + */
> > +enum drm_connector_status msm_dp_bridge_detect(struct drm_bridge *bridge,
> > +					       struct drm_connector *connector)
> > +{
> > +	struct msm_dp_bridge *msm_dp_bridge = to_dp_bridge(bridge);
> > +	struct msm_dp *dp = msm_dp_bridge->msm_dp_display;
> > +	struct msm_dp_display_private *priv;
> > +	int ret = 0;
> 
> First usage is an assignment, so no need for the zero-initialization.
> 
> > +	int status = connector_status_disconnected;
> > +	u8 dpcd[DP_RECEIVER_CAP_SIZE];
> > +	struct drm_dp_desc desc;
> > +
> > +	dp = to_dp_bridge(bridge)->msm_dp_display;
> > +
> > +	priv = container_of(dp, struct msm_dp_display_private, msm_dp_display);
> > +
> > +	if (!dp->link_ready)
> > +		return status;
> 
> So despite the commit message, we're still relying on the link_ready
> flag? (With the improvement that even if the code thinks we've trained
> the link, we can still determine that we should report it as
> disconnected)
> 
> Perhaps I'm missing something here? Did we change the meaning of
> "link_ready"?

Not yet. It changes in the next commit (and I should probably add a
commit renaming it). Note, before the next commit (moving link training)
we can't completely change detect() definition, but we also can't move
link training if we don'g have a proper detect() at that time. I agree
with Jessica's decision here to have two separate commits: this one adds
(imperfect) detect(), the next one moves link training.

> Other than this part, this looks quite familiar to my experiments. Very
> happy to see you continue this work!!!

It has been on my plate for quite a while. Let's finally get it done.

-- 
With best wishes
Dmitry