linux-kernel - Re: [PATCH v1 2/2] drm/msm/dp: add mutex to protect internal_hpd against race condition between different threads

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Message-ID: <6721cc2c-6988-948c-65d6-f50edb925ded@quicinc.com>
Date:   Wed, 10 May 2023 16:19:29 -0700
From:   Kuogee Hsieh <quic_khsieh@...cinc.com>
To:     Abhinav Kumar <quic_abhinavk@...cinc.com>,
        Stephen Boyd <swboyd@...omium.org>, <agross@...nel.org>,
        <airlied@...il.com>, <andersson@...nel.org>, <daniel@...ll.ch>,
        <dianders@...omium.org>, <dmitry.baryshkov@...aro.org>,
        <dri-devel@...ts.freedesktop.org>, <robdclark@...il.com>,
        <sean@...rly.run>, <vkoul@...nel.org>
CC:     <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 2/2] drm/msm/dp: add mutex to protect internal_hpd
 against race condition between different threads

internal_hpd is referenced at both plug and unplug handle.

The majority purpose of  mutext is try to serialize internal_hpd between 
dp_bridge_hpd_disable() and either plug or unplug handle.


On 5/10/2023 4:11 PM, Abhinav Kumar wrote:
>
>
> On 5/10/2023 3:46 PM, Stephen Boyd wrote:
>> Quoting Kuogee Hsieh (2023-05-10 13:31:05)
>>> Intrenal_hpd is referenced by event thread but set by drm bridge 
>>> callback
>>> context. Add mutex to protect internal_hpd to avoid conflicts between
>>> threads.
>>>
>>> Signed-off-by: Kuogee Hsieh <quic_khsieh@...cinc.com>
>>> ---
>>
>> This patch looks completely unnecessary. How can dp_bridge_hpd_enable()
>> be called at the same time that dp_bridge_hpd_disable() is called or
>> dp_bridge_hpd_notify() is called? Isn't there locking or ordering at a
>> higher layer?
>
> Ack. We can drop this patch because we are protected by 
> bridge->hpd_mutex in drm_bridge_hpd_enable() / drm_bridge_hpd_disable 
> () and drm_bridge_hpd_notify().