HDMI Specification Version 1.4b chapter 8.5 mentions:

  An HDMI Sink shall not assert high voltage level on its Hot Plug
  Detect pin when the E-EDID is not available for reading.

  A Source may use a high voltage level Hot Plug Detect signal to
  initiate the reading of E-EDID data.

  An HDMI Sink shall indicate any change to the contents of the E-EDID
  by driving a low voltage level pulse on the Hot Plug Detect pin. This
  pulse shall be at least 100 msec.

Use a work queue to debounce reacting on HPD events to better handle a
HPD low voltage level pulse when a sink changes the EDID.

The 1100 msec hotplug debounce timeout was arbitrarily picked to match
other drivers using same const, and testing using a Raspberry Pi Monitor
seem to use a 200-300 msec pulse when going from standby to power on
state.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
---
v3: New patch
---
 drivers/gpu/drm/bridge/synopsys/dw-hdmi.c | 24 +++++++++++++++++++----
 1 file changed, 20 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c b/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
index f3f8144ae98e..4d079b689b3b 100644
--- a/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
+++ b/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
@@ -51,6 +51,8 @@
 
 #define HDMI14_MAX_TMDSCLK	340000000
 
+#define HOTPLUG_DEBOUNCE_MS	1100
+
 static const u16 csc_coeff_default[3][4] = {
 	{ 0x2000, 0x0000, 0x0000, 0x0000 },
 	{ 0x0000, 0x2000, 0x0000, 0x0000 },
@@ -192,6 +194,7 @@ struct dw_hdmi {
 	hdmi_codec_plugged_cb plugged_cb;
 	struct device *codec_dev;
 	enum drm_connector_status last_connector_result;
+	struct delayed_work hpd_work;
 };
 
 const struct dw_hdmi_plat_data *dw_hdmi_to_plat_data(struct dw_hdmi *hdmi)
@@ -2528,6 +2531,7 @@ static void dw_hdmi_connector_force(struct drm_connector *connector)
 
 	mutex_lock(&hdmi->mutex);
 	hdmi->force = connector->force;
+	hdmi->last_connector_result = connector->status;
 	dw_hdmi_update_phy_mask(hdmi);
 	mutex_unlock(&hdmi->mutex);
 }
@@ -3046,6 +3050,16 @@ void dw_hdmi_setup_rx_sense(struct dw_hdmi *hdmi, bool hpd, bool rx_sense)
 }
 EXPORT_SYMBOL_GPL(dw_hdmi_setup_rx_sense);
 
+static void dw_hdmi_hpd_work(struct work_struct *work)
+{
+	struct dw_hdmi *hdmi = container_of(work, struct dw_hdmi, hpd_work.work);
+
+	if (hdmi->bridge.dev) {
+		drm_helper_hpd_irq_event(hdmi->bridge.dev);
+		drm_bridge_hpd_notify(&hdmi->bridge, hdmi->last_connector_result);
+	}
+}
+
 static irqreturn_t dw_hdmi_irq(int irq, void *dev_id)
 {
 	struct dw_hdmi *hdmi = dev_id;
@@ -3097,10 +3111,8 @@ static irqreturn_t dw_hdmi_irq(int irq, void *dev_id)
 			status == connector_status_connected ?
 			"plugin" : "plugout");
 
-		if (hdmi->bridge.dev) {
-			drm_helper_hpd_irq_event(hdmi->bridge.dev);
-			drm_bridge_hpd_notify(&hdmi->bridge, status);
-		}
+		mod_delayed_work(system_percpu_wq, &hdmi->hpd_work,
+				 msecs_to_jiffies(HOTPLUG_DEBOUNCE_MS));
 	}
 
 	hdmi_writeb(hdmi, intr_stat, HDMI_IH_PHY_STAT0);
@@ -3420,6 +3432,8 @@ struct dw_hdmi *dw_hdmi_probe(struct platform_device *pdev,
 		goto err_res;
 	}
 
+	INIT_DELAYED_WORK(&hdmi->hpd_work, dw_hdmi_hpd_work);
+
 	ret = devm_request_threaded_irq(dev, irq, dw_hdmi_hardirq,
 					dw_hdmi_irq, IRQF_SHARED,
 					dev_name(dev), hdmi);
@@ -3552,6 +3566,8 @@ EXPORT_SYMBOL_GPL(dw_hdmi_probe);
 
 void dw_hdmi_remove(struct dw_hdmi *hdmi)
 {
+	disable_delayed_work_sync(&hdmi->hpd_work);
+
 	drm_bridge_remove(&hdmi->bridge);
 
 	if (hdmi->audio && !IS_ERR(hdmi->audio))
-- 
2.53.0