Re: [PATCH v1] drm/msm/dp: use dp_hpd_plug_handle() and dp_hpd_unplug_handle() directly

From: Abhinav Kumar <quic_abhinavk@quicinc.com>
To: Bjorn Andersson <quic_bjorande@quicinc.com>
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>, <dmitry.baryshkov@linaro.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>
Subject: Re: [PATCH v1] drm/msm/dp: use dp_hpd_plug_handle() and dp_hpd_unplug_handle() directly
Date: Thu, 28 Mar 2024 19:37:42 -0700	[thread overview]
Message-ID: <3bb6722c-5186-6d25-a4a2-c1ef92977dac@quicinc.com> (raw)
In-Reply-To: <20240329014659.GA3478031@hu-bjorande-lv.qualcomm.com>

On 3/28/2024 6:46 PM, Bjorn Andersson wrote:
> On Thu, Mar 28, 2024 at 02:21:14PM -0700, Abhinav Kumar 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.
>>
> 
> It removes the main users of the thread, but there's still code paths
> which will post events on the thread.
> 
> I think I like the direction this is taking, but does it really fix the
> whole problem, or just patch one case?
> 

So kuogee's idea behind this that NON-hpd_isr events need not go through 
event thread at all.

We did not run into any special scenario or issue without this. It was a 
code walkthrough fix.

> 
> PS. Please read go/upstream and switch to b4, to avoid some practical
> issues with the way you posted this patch.
> 
> Thanks,
> Bjorn
> 

Just to elaborate the practical issues so that developers know what you 
encountered:

-> no need of v1 on the PATCH
-> somehow this patch was linked "in-reply-to" another patch 
https://lore.kernel.org/all/1711656246-3483-2-git-send-email-quic_khsieh@quicinc.com/ 
. This is quite strange and not sure how it happened. But will double 
check if we did something wrong here.

Thanks for sharing these.

>>>>
>>>> Looks right to me,
>>>>
>>>> Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>