[PATCH v4 2/2] drm: bridge: cdns-mhdp8546: Add no-hpd property

[Yashas D <y-d@ti.com>]

From: Rahul T R <r-ravikumar@ti.com>

Add a 'no-hpd' boolean property to support boards where the HPD line
cannot be used for hotplug detection due to hardware limitations.

On TI J721S2 EVMs, the DP0 HPD signal has a MUX conflict with audio
functionality. While the HPD pin is physically connected to GPIO0_18,
routing it to the SoC requires sacrificing audio capability due to pin
muxing constraints. This board-level hardware limitation necessitates
an alternative detection mechanism.

When this property is set, the driver uses auxiliary channel (AUX) DPCD
reads to detect monitor presence instead of hardware HPD signals. The
DRM framework polls the connection status via the .detect() callback,
providing hotplug detection without requiring the HPD pin.

Valid use cases:
- HPD pin not routed to connector on PCB
- HPD signal muxed with another function (e.g., audio) on SoC
- Hardware designs where HPD cannot reliably detect monitor presence

Signed-off-by: Rahul T R <r-ravikumar@ti.com>
Signed-off-by: Jayesh Choudhary <j-choudhary@ti.com>
Signed-off-by: Harikrishna Shenoy <h-shenoy@ti.com>
Signed-off-by: Yashas D <y-d@ti.com>
---
 .../drm/bridge/cadence/cdns-mhdp8546-core.c   | 58 ++++++++++++++++---
 .../drm/bridge/cadence/cdns-mhdp8546-core.h   |  1 +
 2 files changed, 52 insertions(+), 7 deletions(-)

diff --git a/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c b/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c
index 36c07b71fe04..5e1bad8fc73e 100644
--- a/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c
+++ b/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c
@@ -53,6 +53,8 @@
 #include "cdns-mhdp8546-hdcp.h"
 #include "cdns-mhdp8546-j721e.h"
 
+static int cdns_mhdp_update_link_status(struct cdns_mhdp_device *mhdp);
+
 static void cdns_mhdp_bridge_hpd_enable(struct drm_bridge *bridge)
 {
 	struct cdns_mhdp_device *mhdp = bridge_to_mhdp(bridge);
@@ -698,7 +700,9 @@ static int cdns_mhdp_fw_activate(const struct firmware *fw,
 	 * MHDP_HW_STOPPED happens only due to driver removal when
 	 * bridge should already be detached.
 	 */
-	cdns_mhdp_bridge_hpd_enable(&mhdp->bridge);
+
+	if (!mhdp->no_hpd)
+		cdns_mhdp_bridge_hpd_enable(&mhdp->bridge);
 
 	spin_unlock(&mhdp->start_lock);
 
@@ -788,9 +792,14 @@ static ssize_t cdns_mhdp_transfer(struct drm_dp_aux *aux,
 		ret = cdns_mhdp_dpcd_read(mhdp, msg->address,
 					  msg->buffer, msg->size);
 		if (ret) {
-			dev_dbg(mhdp->dev,
-				"Failed to read DPCD addr %u\n",
-				msg->address);
+			if (mhdp->no_hpd)
+				dev_dbg(mhdp->dev,
+					"Failed to read DPCD addr %u\n",
+					msg->address);
+			else
+				dev_err(mhdp->dev,
+					"Failed to read DPCD addr %u\n",
+					msg->address);
 
 			return ret;
 		}
@@ -1523,6 +1532,26 @@ static int cdns_mhdp_attach(struct drm_bridge *bridge,
 
 	spin_unlock(&mhdp->start_lock);
 
+	if (mhdp->no_hpd) {
+		/*
+		 * In no-hpd mode there is no interrupt to signal firmware
+		 * readiness. The firmware loads asynchronously after probe(),
+		 * so we must wait here until the uCPU is running before
+		 * attempting the first AUX channel poll for monitor presence.
+		 */
+		ret = wait_event_timeout(mhdp->fw_load_wq,
+					 mhdp->hw_state == MHDP_HW_READY,
+					 msecs_to_jiffies(100));
+		if (ret == 0) {
+			dev_err(mhdp->dev, "%s: Timeout waiting for fw loading\n",
+				__func__);
+			return -ETIMEDOUT;
+		}
+
+		cdns_mhdp_update_link_status(mhdp);
+		return 0;
+	}
+
 	/* Enable SW event interrupts */
 	if (hw_ready)
 		cdns_mhdp_bridge_hpd_enable(bridge);
@@ -2013,6 +2042,9 @@ cdns_mhdp_bridge_detect(struct drm_bridge *bridge, struct drm_connector *connect
 {
 	struct cdns_mhdp_device *mhdp = bridge_to_mhdp(bridge);
 
+	if (mhdp->no_hpd)
+		cdns_mhdp_update_link_status(mhdp);
+
 	return cdns_mhdp_detect(mhdp);
 }
 
@@ -2100,7 +2132,16 @@ static int cdns_mhdp_update_link_status(struct cdns_mhdp_device *mhdp)
 
 	mutex_lock(&mhdp->link_mutex);
 
-	mhdp->plugged = cdns_mhdp_detect_hpd(mhdp, &hpd_pulse);
+	if (mhdp->no_hpd) {
+		ret = drm_dp_dpcd_read_link_status(&mhdp->aux, status);
+		hpd_pulse = false;
+		if (ret < 0)
+			mhdp->plugged = false;
+		else
+			mhdp->plugged = true;
+	} else {
+		mhdp->plugged = cdns_mhdp_detect_hpd(mhdp, &hpd_pulse);
+	}
 
 	if (!mhdp->plugged) {
 		cdns_mhdp_link_down(mhdp);
@@ -2288,6 +2329,8 @@ static int cdns_mhdp_probe(struct platform_device *pdev)
 	mhdp->aux.dev = dev;
 	mhdp->aux.transfer = cdns_mhdp_transfer;
 
+	mhdp->no_hpd = of_property_read_bool(dev->of_node, "no-hpd");
+
 	mhdp->regs = devm_platform_ioremap_resource(pdev, 0);
 	if (IS_ERR(mhdp->regs)) {
 		dev_err(dev, "Failed to get memory resource\n");
@@ -2360,8 +2403,9 @@ static int cdns_mhdp_probe(struct platform_device *pdev)
 	mhdp->display_fmt.bpc = 8;
 
 	mhdp->bridge.of_node = pdev->dev.of_node;
-	mhdp->bridge.ops = DRM_BRIDGE_OP_DETECT | DRM_BRIDGE_OP_EDID |
-			   DRM_BRIDGE_OP_HPD;
+	mhdp->bridge.ops = DRM_BRIDGE_OP_DETECT | DRM_BRIDGE_OP_EDID;
+	if (!mhdp->no_hpd)
+		mhdp->bridge.ops |= DRM_BRIDGE_OP_HPD;
 	mhdp->bridge.type = DRM_MODE_CONNECTOR_DisplayPort;
 
 	ret = phy_init(mhdp->phy);
diff --git a/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.h b/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.h
index b53335b0d22c..24ffb732a207 100644
--- a/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.h
+++ b/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.h
@@ -388,6 +388,7 @@ struct cdns_mhdp_device {
 
 	bool link_up;
 	bool plugged;
+	bool no_hpd;
 
 	/*
 	 * "start_lock" protects the access to bridge_attached and
-- 
2.34.1