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Therefore,
1. If HPD low (which means plugging out) is more than 100ms,
   we need to do de-bouncing (which means we need to wait for 100ms).
2. If HPD low is less than 100ms,
   we don't need to care about the de-bouncing.

In this patch, we can see the timer start to count 100ms and clear the
need_debounce to false.

There will be two situation when HPD high:
1. If the timer is expired (which means it's more than 100ms):
   - need_debounce will be set as true.
   - When HPD high (plugging event comes), need_debounce will be true
     and then we need to do de-bouncing (wait for 100ms).
2. If the timer is not expired (which means it's less than 100ms):
   - need_debounce keeps as false.
   - When HPD high (plugging event comes), need_debounce will be false
     and no need to do de-bouncing.

HPD_______             __________________
          |            |<-  100ms   ->
          |____________|
          <-  100ms   ->

Original issue is that we do not implement the HPD de-bouncing and the
device (Dell Adapter - USB-C to HDMI) will not be detected.
After applying this patch, we can detect the device
(Dell Adapter - USB-C to HDMI) successfully.

We test the following devices and they can be detected successfully:
- Dell Adapter - USB-C to HDMI
- Acer 1in1 HDMI dongle
- Ugreen 1in1 HDMI dongle
- innowatt HDMI + USB3 hub
- Acer 2in1 HDMI dongle
- Apple 3in1 HDMI dongle (A2119)
- J5Create 3in1 HDMI dongle (JAC379)

Tested-by: Rex-BC Chen <rex-bc.chen@xxxxxxxxxxxx>

BRs,
Rex
> 
> Signed-off-by: Jitao Shi <jitao.shi@xxxxxxxxxxxx>
> Signed-off-by: Guillaume Ranquet <granquet@xxxxxxxxxxxx>
> ---
>  drivers/gpu/drm/mediatek/mtk_dp.c | 23 +++++++++++++++++++++++
>  1 file changed, 23 insertions(+)
> 
> diff --git a/drivers/gpu/drm/mediatek/mtk_dp.c
> b/drivers/gpu/drm/mediatek/mtk_dp.c
> index 2a3d5f15b651b..fe91ab8b2fd89 100644
> --- a/drivers/gpu/drm/mediatek/mtk_dp.c
> +++ b/drivers/gpu/drm/mediatek/mtk_dp.c
> @@ -178,6 +178,8 @@ struct mtk_dp {
>  	struct device *codec_dev;
>  	u8 connector_eld[MAX_ELD_BYTES];
>  	struct drm_connector *conn;
> +	bool need_debounce;
> +	struct timer_list debounce_timer;
>  };
>  
>  static struct regmap_config mtk_dp_regmap_config = {
> @@ -1698,6 +1700,9 @@ static irqreturn_t mtk_dp_hpd_event_thread(int
> hpd, void *dev)
>  	if (event < 0)
>  		return IRQ_HANDLED;
>  
> +	if (mtk_dp->need_debounce && mtk_dp-
> >train_info.cable_plugged_in)
> +		msleep(100);
> +
>  	if (mtk_dp->drm_dev) {
>  		dev_info(mtk_dp->dev, "drm_helper_hpd_irq_event\n");
>  		drm_helper_hpd_irq_event(mtk_dp->bridge.dev);
> @@ -1776,6 +1781,13 @@ static irqreturn_t
> mtk_dp_hpd_isr_handler(struct mtk_dp *mtk_dp)
>  	}
>  	train_info->cable_state_change = true;
>  
> +	if (train_info->cable_state_change) {
> +		if (!train_info->cable_plugged_in) {
> +			mod_timer(&mtk_dp->debounce_timer, jiffies +
> msecs_to_jiffies(100) - 1);
> +			mtk_dp->need_debounce = false;
> +		}
> +	}
> +
>  	return IRQ_WAKE_THREAD;
>  }
>  
> @@ -2239,6 +2251,13 @@ static const struct drm_bridge_funcs
> mtk_dp_bridge_funcs = {
>  	.detect = mtk_dp_bdg_detect,
>  };
>  
> +static void mtk_dp_debounce_timer(struct timer_list *t)
> +{
> +	struct mtk_dp *mtk_dp = from_timer(mtk_dp, t, debounce_timer);
> +
> +	mtk_dp->need_debounce = true;
> +}
> +
>  static int mtk_dp_probe(struct platform_device *pdev)
>  {
>  	struct mtk_dp *mtk_dp;
> @@ -2319,6 +2338,9 @@ static int mtk_dp_probe(struct platform_device
> *pdev)
>  	else
>  		mtk_dp->bridge.type = DRM_MODE_CONNECTOR_DisplayPort;
>  
> +	mtk_dp->need_debounce = true;
> +	timer_setup(&mtk_dp->debounce_timer, mtk_dp_debounce_timer, 0);
> +
>  	pm_runtime_enable(dev);
>  	pm_runtime_get_sync(dev);
>  
> @@ -2332,6 +2354,7 @@ static int mtk_dp_remove(struct platform_device
> *pdev)
>  	platform_device_unregister(mtk_dp->phy_dev);
>  
>  	mtk_dp_video_mute(mtk_dp, true);
> +	del_timer_sync(&mtk_dp->debounce_timer);
>  
>  	pm_runtime_disable(&pdev->dev);
>  




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