linux-kernel - Re: [PATCH 1/2] drm/msm/dp: service only one irq_hpd if there are multiple irq

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Date:   Wed, 21 Apr 2021 10:26:21 -0700
From:   khsieh@...eaurora.org
To:     Stephen Boyd <swboyd@...omium.org>
Cc:     robdclark@...il.com, sean@...rly.run, abhinavk@...eaurora.org,
        aravindh@...eaurora.org, airlied@...ux.ie, daniel@...ll.ch,
        linux-arm-msm@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        freedreno@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] drm/msm/dp: service only one irq_hpd if there are
 multiple irq_hpd pending

On 2021-04-20 15:01, Stephen Boyd wrote:
> Quoting Kuogee Hsieh (2021-04-16 13:27:57)
>> Some dongle may generate more than one irq_hpd events in a short 
>> period of
>> time. This patch will treat those irq_hpd events as single one and 
>> service
>> only one irq_hpd event.
> 
> Why is it bad to get multiple irq_hpd events in a short period of time?
> Please tell us here in the commit text.
> 
>> 
>> Signed-off-by: Kuogee Hsieh <khsieh@...eaurora.org>
>> ---
>>  drivers/gpu/drm/msm/dp/dp_display.c | 9 +++++++++
>>  1 file changed, 9 insertions(+)
>> 
>> diff --git a/drivers/gpu/drm/msm/dp/dp_display.c 
>> b/drivers/gpu/drm/msm/dp/dp_display.c
>> index 5a39da6..0a7d383 100644
>> --- a/drivers/gpu/drm/msm/dp/dp_display.c
>> +++ b/drivers/gpu/drm/msm/dp/dp_display.c
>> @@ -707,6 +707,9 @@ static int dp_irq_hpd_handle(struct 
>> dp_display_private *dp, u32 data)
>>                 return 0;
>>         }
>> 
>> +       /* only handle first irq_hpd in case of multiple irs_hpd 
>> pending */
>> +       dp_del_event(dp, EV_IRQ_HPD_INT);
>> +
>>         ret = dp_display_usbpd_attention_cb(&dp->pdev->dev);
>>         if (ret == -ECONNRESET) { /* cable unplugged */
>>                 dp->core_initialized = false;
>> @@ -1300,6 +1303,9 @@ static int dp_pm_suspend(struct device *dev)
>>         /* host_init will be called at pm_resume */
>>         dp->core_initialized = false;
>> 
>> +       /* system suspended, delete pending irq_hdps */
>> +       dp_del_event(dp, EV_IRQ_HPD_INT);
> 
> What happens if I suspend my device and when this function is running I
> toggle my monitor to use the HDMI input that is connected instead of 
> some
> other input, maybe the second HDMI input? Wouldn't that generate an HPD
> interrupt to grab the attention of this device?
no,
At this time display is off. this mean dp controller is off and mainlink 
has teared down.
it will start with plug in interrupt to bring dp controller up and start 
link training.
irq_hpd can be generated only panel is at run time of operation mode and 
need attention from host.
If host is shutting down, then no need to service pending irq_hpd.

> 
>> +
>>         mutex_unlock(&dp->event_mutex);
>> 
>>         return 0;
>> @@ -1496,6 +1502,9 @@ int msm_dp_display_disable(struct msm_dp *dp, 
>> struct drm_encoder *encoder)
>>         /* stop sentinel checking */
>>         dp_del_event(dp_display, EV_DISCONNECT_PENDING_TIMEOUT);
>> 
>> +       /* link is down, delete pending irq_hdps */
>> +       dp_del_event(dp_display, EV_IRQ_HPD_INT);
>> +
> 
> I'm becoming convinced that the whole kthread design and event queue is
> broken. These sorts of patches are working around the larger problem
> that the kthread is running independently of the driver and irqs can
> come in at any time but the event queue is not checked from the irq
> handler to debounce the irq event. Is the event queue necessary at all?
> I wonder if it would be simpler to just use an irq thread and process
> the hpd signal from there. Then we're guaranteed to not get an irq 
> again
> until the irq thread is done processing the event. This would naturally
> debounce the irq hpd event that way.
event q just like bottom half of irq handler. it turns irq into event 
and handle them sequentially.
irq_hpd is asynchronous event from panel to bring up attention of hsot 
during run time of operation.
Here, the dongle is unplugged and main link had teared down so that no 
need to service pending irq_hpd if any.


> 
>>         dp_display_disable(dp_display, 0);
>> 
>>         rc = dp_display_unprepare(dp);