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Date: Thu, 28 Mar 2024 22:47:03 -0700
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Subject: Re: [PATCH v1] drm/msm/dp: use dp_hpd_plug_handle() and
 dp_hpd_unplug_handle() directly
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To: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
CC: Stephen Boyd <swboyd@chromium.org>,
        Bjorn Andersson
	<andersson@kernel.org>,
        Johan Hovold <johan@kernel.org>,
        Kuogee Hsieh
	<quic_khsieh@quicinc.com>, <abel.vesa@linaro.org>,
        <agross@kernel.org>, <airlied@gmail.com>, <daniel@ffwll.ch>,
        <dianders@chromium.org>, <dri-devel@lists.freedesktop.org>,
        <robdclark@gmail.com>, <sean@poorly.run>, <vkoul@kernel.org>,
        <quic_jesszhan@quicinc.com>, <quic_sbillaka@quicinc.com>,
        <marijn.suijten@somainline.org>, <freedreno@lists.freedesktop.org>,
        <linux-arm-msm@vger.kernel.org>, <linux-kernel@vger.kernel.org>
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From: Abhinav Kumar <quic_abhinavk@quicinc.com>
In-Reply-To: <CAA8EJprES3q3w6GuQ8dW5vicnzbO_eZ6wpNfWe5njTPRDZm5KQ@mail.gmail.com>
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On 3/28/2024 8:23 PM, Dmitry Baryshkov wrote:
> On Fri, 29 Mar 2024 at 04:16, Abhinav Kumar <quic_abhinavk@quicinc.com> wrote:
>>
>>
>>
>> On 3/28/2024 5:10 PM, Dmitry Baryshkov wrote:
>>> On Fri, 29 Mar 2024 at 01:42, Abhinav Kumar <quic_abhinavk@quicinc.com> wrote:
>>>>
>>>>
>>>>
>>>> On 3/28/2024 3:50 PM, Dmitry Baryshkov wrote:
>>>>> On Thu, 28 Mar 2024 at 23:21, Abhinav Kumar <quic_abhinavk@quicinc.com> wrote:
>>>>>>
>>>>>>
>>>>>>
>>>>>> On 3/28/2024 1:58 PM, Stephen Boyd wrote:
>>>>>>> Quoting Abhinav Kumar (2024-03-28 13:24:34)
>>>>>>>> + Johan and Bjorn for FYI
>>>>>>>>
>>>>>>>> On 3/28/2024 1:04 PM, Kuogee Hsieh wrote:
>>>>>>>>> For internal HPD case, hpd_event_thread is created to handle HPD
>>>>>>>>> interrupts generated by HPD block of DP controller. It converts
>>>>>>>>> HPD interrupts into events and executed them under hpd_event_thread
>>>>>>>>> context. For external HPD case, HPD events is delivered by way of
>>>>>>>>> dp_bridge_hpd_notify() under thread context. Since they are executed
>>>>>>>>> under thread context already, there is no reason to hand over those
>>>>>>>>> events to hpd_event_thread. Hence dp_hpd_plug_handle() and
>>>>>>>>> dp_hpd_unplug_hanlde() are called directly at dp_bridge_hpd_notify().
>>>>>>>>>
>>>>>>>>> Signed-off-by: Kuogee Hsieh <quic_khsieh@quicinc.com>
>>>>>>>>> ---
>>>>>>>>>       drivers/gpu/drm/msm/dp/dp_display.c | 5 +++--
>>>>>>>>>       1 file changed, 3 insertions(+), 2 deletions(-)
>>>>>>>>>
>>>>>>>>
>>>>>>>> Fixes: 542b37efc20e ("drm/msm/dp: Implement hpd_notify()")
>>>>>>>
>>>>>>> Is this a bug fix or an optimization? The commit text doesn't tell me.
>>>>>>>
>>>>>>
>>>>>> I would say both.
>>>>>>
>>>>>> optimization as it avoids the need to go through the hpd_event thread
>>>>>> processing.
>>>>>>
>>>>>> bug fix because once you go through the hpd event thread processing it
>>>>>> exposes and often breaks the already fragile hpd handling state machine
>>>>>> which can be avoided in this case.
>>>>>
>>>>> Please add a description for the particular issue that was observed
>>>>> and how it is fixed by the patch.
>>>>>
>>>>> Otherwise consider there to be an implicit NAK for all HPD-related
>>>>> patches unless it is a series that moves link training to the enable
>>>>> path and drops the HPD state machine completely.
>>>>>
>>>>> I really mean it. We should stop beating a dead horse unless there is
>>>>> a grave bug that must be fixed.
>>>>>
>>>>
>>>> I think the commit message is explaining the issue well enough.
>>>>
>>>> This was not fixing any issue we saw to explain you the exact scenario
>>>> of things which happened but this is just from code walkthrough.
>>>>
>>>> Like kuogee wrote, hpd event thread was there so handle events coming
>>>> out of the hpd_isr for internal hpd cases. For the hpd_notify coming
>>>> from pmic_glink or any other extnernal hpd cases, there is no need to
>>>> put this through the hpd event thread because this will only make things
>>>> worse of exposing the race conditions of the state machine.
>>>>
>>>> Moving link training to enable and removal of hpd event thread will be
>>>> worked on but delaying obvious things we can fix does not make sense.
>>>
>>>   From the commit message this feels like an optimisation rather than a
>>> fix. And granted the fragility of the HPD state machine, I'd prefer to
>>> stay away from optimisations. As far as I understood from the history
>>> of the last revert, we'd better make sure that HPD handling goes only
>>> through the HPD event thread.
>>>
>>
>> I think you are mixing the two. We tried to send the events through
>> DRM's hpd_notify which ended up in a bad way and btw, thats still not
>> resolved even though I have seen reports that things are fine with the
>> revert, we are consistently able to see us ending up in a disconnected
>> state with all the reverts and fixes in our x1e80100 DP setup.
>>
>> I plan to investigate that issue properly in the next week and try to
>> make some sense of it all.
>>
>> In fact, this patch is removing one more user of the hpd event thread
>> which is the direction in which we all want to head towards.
> 
> As I stated earlier, from my point of view it doesn't make sense to
> rework the HPD thread in small steps.
> 
>> On whether this is an optimization or a bug fix. I think by avoiding hpd
>> event thread (which should have never been used for hpd_notify updates,
>> hence a bug) we are avoiding the possibility of more race conditions.
> 
> I think that the HPD event thread serializes handling of events, so
> avoiding it increases the possibility of a race condition.
> 
>>
>> So, this has my R-b and it holds. Upto you.
> 
> I'd wait for a proper description of the issue that was observed and
> how it is solved by this patch.
> 

This was a code walkthrough fix as I wrote a few times. If there no 
merit in pushing this, lets ignore it and stop discussing.