linux-kernel - Re: [PATCH v2 26/38] drm/msm/dp: skip reading the EDID for MST cases

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Message-ID: <pbbisppntd7fntghbjbyjmdvbxqc7uux42kiuryau4trgf3w47@4d5fi6zpcdf5>
Date: Thu, 14 Aug 2025 12:27:11 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
To: Yongxing Mou <quic_yongmou@...cinc.com>
Cc: Rob Clark <robin.clark@....qualcomm.com>,
        Abhinav Kumar <abhinav.kumar@...ux.dev>,
        Jessica Zhang <jessica.zhang@....qualcomm.com>,
        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,
        Abhinav Kumar <quic_abhinavk@...cinc.com>
Subject: Re: [PATCH v2 26/38] drm/msm/dp: skip reading the EDID for MST cases

On Thu, Aug 14, 2025 at 04:22:44PM +0800, Yongxing Mou wrote:
> 
> 
> On 2025/6/9 23:58, Dmitry Baryshkov wrote:
> > On Mon, Jun 09, 2025 at 08:21:45PM +0800, Yongxing Mou wrote:
> > > From: Abhinav Kumar <quic_abhinavk@...cinc.com>
> > > 
> > > For MST cases, EDID is handled through AUX sideband messaging.
> > > Skip the EDID read during hotplug handle for MST cases.
> > 
> > Why? It makes sense to read it during the HPD processing, ping HDMI
> > codec, update CEC info, etc.
> > 
> For MST case to read EDID. we will use drm_dp_mst_edid_read when MST
> connetors .get_modes() called.

I see that other MST drivers indeed read EDID from get_modes(). The only
issue that I can foresee is the audio handling. We've discussed that
some time ago (for the HDMI implementation) and it's generally expected
that we notify ASoC (and provide ELD) from the HPD (detect) path rather
than the get_modes() or e.g. atomic_enable().

Note: AMD / Radeon, Nouveau and Intel drivers don't have this problem
since they don't use ASoC and hdmi_codec ops.

> > > 
> > > Signed-off-by: Abhinav Kumar <quic_abhinavk@...cinc.com>
> > > Signed-off-by: Yongxing Mou <quic_yongmou@...cinc.com>
> > > ---
> > >   drivers/gpu/drm/msm/dp/dp_display.c | 8 +++++---
> > >   1 file changed, 5 insertions(+), 3 deletions(-)
> > > 
> > > diff --git a/drivers/gpu/drm/msm/dp/dp_display.c b/drivers/gpu/drm/msm/dp/dp_display.c
> > > index 88cae0ca66015377e59bee757462edeae5ae91bf..b1b025d1d356046f8f9e3d243fc774185df24318 100644
> > > --- a/drivers/gpu/drm/msm/dp/dp_display.c
> > > +++ b/drivers/gpu/drm/msm/dp/dp_display.c
> > > @@ -438,9 +438,11 @@ static int msm_dp_display_process_hpd_high(struct msm_dp_display_private *dp)
> > >   	if (rc)
> > >   		goto end;
> > > -	rc = msm_dp_panel_read_edid(dp->panel, connector);
> > > -	if (rc)
> > > -		goto end;
> > > +	if (!dp->mst_supported || !drm_dp_read_mst_cap(dp->aux, dp->panel->dpcd)) {
> > > +		rc = msm_dp_panel_read_edid(dp->panel, connector);
> > > +		if (rc)
> > > +			goto end;
> > > +	}
> > >   	msm_dp_link_process_request(dp->link);
> > > 
> > > -- 
> > > 2.34.1
> > > 
> > 
> 

-- 
With best wishes
Dmitry