From: Jitao Shi <redacted>

From the DP spec 1.4a chapter 3.3, upstream devices should implement
HPD signal de-bouncing on an external connection.
A period of 100ms should be used to detect an HPD connect event.
To cover these cases, HPD de-bounce should be implemented only after
HPD low has been detected for at least 100ms.

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

In this patch, we start a 100ms timer and use a need_debounce boolean
to implement the feature.

Two cases when HPD is high:
1. If the timer is expired (>100ms):
   - need_debounce is 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 (<100ms):
   - need_debounce is false.
   - When HPD high (plugging event comes), need_debounce will be false
     and no need to do de-bouncing.

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

Without HPD de-bouncing, USB-C to HDMI Adapaters will not be detected.

The change has been successfully tested with the following devices:
- 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 <redacted>
Reviewed-by: Rex-BC Chen <redacted>
Signed-off-by: Jitao Shi <redacted>
Signed-off-by: Guillaume Ranquet <redacted>
Signed-off-by: Bo-Chen Chen <redacted>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
---
 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 610651dd6365..50a4159095a1 100644
--- a/drivers/gpu/drm/mediatek/mtk_dp.c
+++ b/drivers/gpu/drm/mediatek/mtk_dp.c

@@ -105,6 +105,8 @@ struct mtk_dp {
 	bool enabled;
 
 	struct drm_connector *conn;
+	bool need_debounce;
+	struct timer_list debounce_timer;
 };
 
 static struct regmap_config mtk_dp_regmap_config = {

@@ -1551,7 +1553,11 @@ 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);
+
 	dev_dbg(mtk_dp->dev, "drm_helper_hpd_irq_event\n");
+
 	drm_helper_hpd_irq_event(mtk_dp->bridge.dev);
 
 	if (mtk_dp->train_info.cable_state_change) {

@@ -1614,6 +1620,12 @@ static irqreturn_t mtk_dp_hpd_isr_handler(struct mtk_dp *mtk_dp)
 
 	train_info->cable_state_change = true;
 
+	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;
 }
 

@@ -2058,6 +2070,13 @@ 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;

@@ -2135,6 +2154,9 @@ static int mtk_dp_probe(struct platform_device *pdev)
 
 	drm_bridge_add(&mtk_dp->bridge);
 
+	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);
 

@@ -2147,6 +2169,7 @@ 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);
 

-- 
2.18.0