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

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Message-ID: <CAA8EJpp6Lc7sc5fnKp+O8TYdaywiE+dZ=YJin351s=r5rxi+Kw@mail.gmail.com>
Date: Tue, 9 Apr 2024 01:07:57 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To: Abhinav Kumar <quic_abhinavk@...cinc.com>
Cc: Bjorn Andersson <andersson@...nel.org>, 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 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
> >>>> index d80f89581760..bfb6dfff27e8 100644
> >>>> --- a/drivers/gpu/drm/msm/dp/dp_display.c
> >>>> +++ b/drivers/gpu/drm/msm/dp/dp_display.c
> >>>> @@ -1665,7 +1665,7 @@ void dp_bridge_hpd_notify(struct drm_bridge *bridge,
> >>>>               return;
> >>>>
> >>>>       if (!dp_display->link_ready && status == connector_status_connected)
> >>>> -            dp_add_event(dp, EV_HPD_PLUG_INT, 0, 0);
> >>>> +            dp_hpd_plug_handle(dp, 0);
> >>>
> >>> If I read the code correctly, and we get an external connect event
> >>> inbetween a previous disconnect and the related disable call, this
> >>> should result in a PLUG_INT being injected into the queue still.
> >>>
> >>> Will that not cause the same problem?
> >>>
> >>> Regards,
> >>> Bjorn
> >>>
> >>
> >> Yes, your observation is correct and I had asked the same question to
> >> kuogee before taking over this change and posting.
> >
> > Should it then have the Co-developed-by trailers?
> >
>
> hmmm, perhaps but that didnt result in any code change between v2 and
> v3, so I didnt add it.

So who authored v0 of it? From your text I had an impression that it
was Kuogee. Please excuse me if I was wrong.

>
> >> We will have to handle that case separately. I don't have a good
> >> solution yet for it without requiring further rework or we drop the
> >> below snippet.
> >>
> >>           if (state == ST_DISCONNECT_PENDING) {
> >>                   /* wait until ST_DISCONNECTED */
> >>                   dp_add_event(dp, EV_HPD_PLUG_INT, 0, 1); /* delay = 1 */
> >>                   mutex_unlock(&dp->event_mutex);
> >>                   return 0;
> >>           }
> >>
> >> 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.

>
> So we need to do both, make .detect() return correct status AND drop hpd
> event thread processing.
>
> But, dropping the hpd event thread processing alone was fixing the
> complimentary events issue. So kuogee had only this part in this patch.

I'd prefer to wait for the final patchset then. Which has the HPD
thread completely removed.

>
>
> >>>>       else if (dp_display->link_ready && status == connector_status_disconnected)
> >>>> -            dp_add_event(dp, EV_HPD_UNPLUG_INT, 0, 0);
> >>>> +            dp_hpd_unplug_handle(dp, 0);
> >>>>    }
> >>>> --
> >>>> 2.43.2
> >>>>
> >
> >
> >



--
With best wishes
Dmitry