linux-kernel - Re: [PATCH v3] drm/msm/dp: call dp_hpd_plug_handle()/unplug

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Date: Tue, 9 Apr 2024 17:39:37 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To: Bjorn Andersson <andersson@...nel.org>
Cc: Abhinav Kumar <quic_abhinavk@...cinc.com>, 
	freedreno@...ts.freedesktop.org, Rob Clark <robdclark@...il.com>, Sean Paul <sean@...rly.run>, 
	Marijn Suijten <marijn.suijten@...ainline.org>, David Airlie <airlied@...il.com>, 
	Daniel Vetter <daniel@...ll.ch>, Kuogee Hsieh <quic_khsieh@...cinc.com>, 
	dri-devel@...ts.freedesktop.org, seanpaul@...omium.org, swboyd@...omium.org, 
	quic_jesszhan@...cinc.com, quic_bjorande@...cinc.com, johan@...nel.org, 
	linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] drm/msm/dp: call dp_hpd_plug_handle()/unplug_handle()
 directly for external HPD

On Mon, Apr 08, 2024 at 09:33:01PM -0500, Bjorn Andersson wrote:
> On Tue, Apr 09, 2024 at 01:07:57AM +0300, Dmitry Baryshkov wrote:
> > On Tue, 9 Apr 2024 at 00:23, Abhinav Kumar <quic_abhinavk@...cinc.com> wrote:
> > > On 4/8/2024 2:12 PM, Dmitry Baryshkov wrote:
> > > > On Mon, 8 Apr 2024 at 22:43, Abhinav Kumar <quic_abhinavk@...cinc.com> wrote:
> > > >> On 4/7/2024 11:48 AM, Bjorn Andersson wrote:
> > > >>> On Fri, Apr 05, 2024 at 08:15:47PM -0700, Abhinav Kumar wrote:
> > > >>>> From: Kuogee Hsieh <quic_khsieh@...cinc.com>
> > > >>> [..]
> > > >>>> diff --git a/drivers/gpu/drm/msm/dp/dp_display.c b/drivers/gpu/drm/msm/dp/dp_display.c
> [..]
> > > >>
> > > >> I will need sometime to address that use-case as I need to see if we can
> > > >> handle that better and then drop the the DISCONNECT_PENDING state to
> > > >> address this fully. But it needs more testing.
> > > >>
> > > >> But, we will need this patch anyway because without this we will not be
> > > >> able to fix even the most regular and commonly seen case of basic
> > > >> connect/disconnect receiving complementary events.
> > > >
> > > > Hmm, no. We need to drop the HPD state machine, not to patch it. Once
> > > > the driver has proper detect() callback, there will be no
> > > > complementary events. That is a proper way to fix the code, not these
> > > > kinds of band-aids patches.
> > > >
> > >
> > > I had discussed this part too :)
> > >
> > > I totally agree we should fix .detect()'s behavior to just match cable
> > > connect/disconnect and not link_ready status.
> > >
> > > But that alone would not have fixed this issue. If HPD thread does not
> > > get scheduled and plug_handle() was not executed, .detect() would have
> > > still returned old status as we will update the cable status only in
> > > plug_handle() / unplug_handle() to have a common API between internal
> > > and external hpd execution.
> > 
> > I think there should be just hpd_notify, which if the HPD is up,
> > attempts to read the DPCD. No need for separate plug/unplug_handle.
> > The detect() can be as simple as !drm_dp_is_branch() || sink_count != 0.
> > 
> 
> What is detect() supposed to return in the event that we have external
> HPD handler? The link state? While the external HPD bridge would return
> the HPD state?

It should return the same: there is a sensible display attached. Other
drivers (and drm/msm/dp internally) use !branch || (sink_count > 0).

> If a driver only drives the link inbetween atomic_enable() and
> atomic_disable() will the "connected state" then ever be reported as
> "connected"? (I'm sure I'm still missing pieces of this puzzle).

I don't probably get the question. Nothing stops the driver from
accessing the AUX bus outside of the atomic_enable/disable() brackets.

> 
> Regards,
> Bjorn

-- 
With best wishes
Dmitry