Date: Fri, 5 Apr 2024 17:04:03 -0700
From: Abhinav Kumar <quic_abhinavk@...cinc.com>
To: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
CC: Stephen Boyd <swboyd@...omium.org>,
        Bjorn Andersson
	<andersson@...nel.org>,
        Johan Hovold <johan@...nel.org>,
        Kuogee Hsieh
	<quic_khsieh@...cinc.com>, <abel.vesa@...aro.org>,
        <agross@...nel.org>, <airlied@...il.com>, <daniel@...ll.ch>,
        <dianders@...omium.org>, <dri-devel@...ts.freedesktop.org>,
        <robdclark@...il.com>, <sean@...rly.run>, <vkoul@...nel.org>,
        <quic_jesszhan@...cinc.com>, <quic_sbillaka@...cinc.com>,
        <marijn.suijten@...ainline.org>, <freedreno@...ts.freedesktop.org>,
        <linux-arm-msm@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1] drm/msm/dp: use dp_hpd_plug_handle() and
 dp_hpd_unplug_handle() directly



On 3/28/2024 10:47 PM, Abhinav Kumar wrote:
> 
> 
> On 3/28/2024 8:23 PM, Dmitry Baryshkov wrote:
>> On Fri, 29 Mar 2024 at 04:16, Abhinav Kumar 
>> <quic_abhinavk@...cinc.com> wrote:
>>>
>>>
>>>
>>> On 3/28/2024 5:10 PM, Dmitry Baryshkov wrote:
>>>> On Fri, 29 Mar 2024 at 01:42, Abhinav Kumar 
>>>> <quic_abhinavk@...cinc.com> wrote:
>>>>>
>>>>>
>>>>>
>>>>> On 3/28/2024 3:50 PM, Dmitry Baryshkov wrote:
>>>>>> On Thu, 28 Mar 2024 at 23:21, Abhinav Kumar 
>>>>>> <quic_abhinavk@...cinc.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@...cinc.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.
> 

Ok, so after we debugged the HPD issue on we have found the issue and 
why actually this change will help. I am going to post a V2 with more 
details on the commit text. We can discuss after that.

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