linux-kernel - Re: [PATCH 18/19] drm/msm/dp: Move link training to atomic

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Message-ID: <qz725rrbpae5iw6tmx2s2pbaxl7pliyxf7dd3gyev64zqrbq5t@aeacyijbswl3>
Date: Sat, 2 Aug 2025 13:18:53 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
To: Jessica Zhang <jessica.zhang@....qualcomm.com>
Cc: Rob Clark <robin.clark@....qualcomm.com>,
        Abhinav Kumar <abhinav.kumar@...ux.dev>,
        Dmitry Baryshkov <lumag@...nel.org>, Sean Paul <sean@...rly.run>,
        Marijn Suijten <marijn.suijten@...ainline.org>,
        David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
        linux-arm-msm@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        freedreno@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Yongxing Mou <quic_yongmou@...cinc.com>
Subject: Re: [PATCH 18/19] drm/msm/dp: Move link training to atomic_enable()

On Fri, Aug 01, 2025 at 04:58:55PM -0700, Jessica Zhang wrote:
> 
> 
> On 7/14/2025 4:54 AM, Dmitry Baryshkov wrote:
> > On Fri, Jul 11, 2025 at 05:58:23PM -0700, Jessica Zhang wrote:
> > > Currently, the DP link training is being done during HPD. Move
> > > link training to atomic_enable() in accordance with the atomic_enable()
> > > documentation.
> > > 
> > > In addition, don't disable the link until atomic_post_disable() (as part
> > > of the dp_ctrl_off[_link_stream]() helpers).
> > > 
> > > Since the link training is moved to a later part of the enable sequence,
> > > change the bridge detect() to return true when the display is physically
> > > connected instead of when the link is ready.
> > 
> > These two parts should be patch #2 in the series.
> > 
> > > 
> > > Finally, call the plug/unplug handlers directly in hpd_notify() instead
> > > of queueing them in the event thread so that they aren't preempted by
> > > other events.
> > > 
> > > Signed-off-by: Jessica Zhang <jessica.zhang@....qualcomm.com>
> > > ---
> > >   drivers/gpu/drm/msm/dp/dp_display.c | 15 ++++++++-------
> > >   drivers/gpu/drm/msm/dp/dp_drm.c     |  6 +++---
> > >   2 files changed, 11 insertions(+), 10 deletions(-)
> > > 
> > > diff --git a/drivers/gpu/drm/msm/dp/dp_display.c b/drivers/gpu/drm/msm/dp/dp_display.c
> > > index 87f2750a99ca..32e1ee40c2c3 100644
> > > --- a/drivers/gpu/drm/msm/dp/dp_display.c
> > > +++ b/drivers/gpu/drm/msm/dp/dp_display.c
> > > @@ -410,11 +410,6 @@ static int msm_dp_display_process_hpd_high(struct msm_dp_display_private *dp)
> > >   	msm_dp_link_psm_config(dp->link, &dp->panel->link_info, false);
> > >   	msm_dp_link_reset_phy_params_vx_px(dp->link);
> > > -	rc = msm_dp_ctrl_on_link(dp->ctrl);
> > > -	if (rc) {
> > > -		DRM_ERROR("failed to complete DP link training\n");
> > > -		goto end;
> > > -	}
> > >   	msm_dp_add_event(dp, EV_USER_NOTIFICATION, true, 0);
> > > @@ -1561,6 +1556,12 @@ void msm_dp_bridge_atomic_enable(struct drm_bridge *drm_bridge,
> > >   		force_link_train = true;
> > >   	}
> > > +	rc = msm_dp_ctrl_on_link(msm_dp_display->ctrl);
> > > +	if (rc) {
> > > +		DRM_ERROR("Failed link training (rc=%d)\n", rc);
> > > +		dp->connector->state->link_status = DRM_LINK_STATUS_BAD;
> > > +	}
> > > +
> > >   	msm_dp_display_enable(msm_dp_display, force_link_train);
> > >   	rc = msm_dp_display_post_enable(dp);
> > > @@ -1706,7 +1707,7 @@ void msm_dp_bridge_hpd_notify(struct drm_bridge *bridge,
> > >   		return;
> > >   	if (!msm_dp_display->link_ready && status == connector_status_connected)
> > > -		msm_dp_add_event(dp, EV_HPD_PLUG_INT, 0, 0);
> > > +		msm_dp_hpd_plug_handle(dp, 0);
> > >   	else if (msm_dp_display->link_ready && status == connector_status_disconnected)
> > > -		msm_dp_add_event(dp, EV_HPD_UNPLUG_INT, 0, 0);
> > > +		msm_dp_hpd_unplug_handle(dp, 0);
> > 
> > This chunk should be separated from this patch. I'd ask to drop
> > EV_HPD_PLUG_INT / EV_HPD_UNPLUG_INT completely and call DRM functions
> > all over the place instead. You can do it in a single patch, which comes
> > after this one.
> 
> Hi Dmitry,
> 
> Sure I can split this into a separate patch.
> 
> Is the goal here to remove the event queue entirely?

I think so.

> 
> I can drop EV_USER_NOTIFICATION,

With the link training being moved to atomic_enable, there should be no
need for an extra event here, I agree.

> but I'm not sure if I can completely drop
> EV_HPD_[UN]PLUG_INT entirely without major refactor of the plug/unplug
> handlers since they are used for the HPD IRQ handling.

And one of the pieces of the problem is that it's not doing its job
correctly.

The code flow should be:
- Inside the IRQ handler notify DRM core about HPD events from the
  bridge, don't do anything else.
- Inside detect() callback read DPCD bits and identify if there is a
  valid branch device.
- Inside hpd_notify() check if DPRX has sent IRQ_HPD pulse, handle the
  rest of the tasks: link events, etc.

Note: we might want to duplicate DPCD reading between detect() and
hpd_notify() in order to relieve detect from updating the DP structures.

-- 
With best wishes
Dmitry