[PATCH v6 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 resistor is not populated from factory
(DNI), so the HPD signal is not physically connected to SoC pin AA24
by default which makes HPD unavailable but AA24 must be in DP0_HPD
mux mode for the MHDP firmware to operate.

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 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   | 79 +++++++++++++++++--
 .../drm/bridge/cadence/cdns-mhdp8546-core.h   |  1 +
 2 files changed, 72 insertions(+), 8 deletions(-)

diff --git a/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c b/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c
index 504a3186ebb3..ae9bbec855f3 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);
 
@@ -739,7 +743,13 @@ static void cdns_mhdp_fw_cb(const struct firmware *fw, void *context)
 	spin_lock(&mhdp->start_lock);
 	bridge_attached = mhdp->bridge_attached;
 	spin_unlock(&mhdp->start_lock);
-	if (bridge_attached)
+
+	if (!bridge_attached)
+		return;
+
+	if (mhdp->no_hpd)
+		cdns_mhdp_update_link_status(mhdp);
+	else
 		drm_bridge_hpd_notify(&mhdp->bridge, cdns_mhdp_detect(mhdp));
 }
 
@@ -788,9 +798,14 @@ 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 +1538,19 @@ static int cdns_mhdp_attach(struct drm_bridge *bridge,
 
 	spin_unlock(&mhdp->start_lock);
 
+	if (mhdp->no_hpd) {
+		/*
+		 * In no-hpd mode there are no HPD interrupts to trigger
+		 * detection. If firmware is already ready, do the initial
+		 * AUX poll immediately. Otherwise fw_cb() will call
+		 * cdns_mhdp_update_link_status() once firmware finishes
+		 * loading and sees bridge_attached is true.
+		 */
+		if (hw_ready)
+			cdns_mhdp_update_link_status(mhdp);
+		return 0;
+	}
+
 	/* Enable SW event interrupts */
 	if (hw_ready)
 		cdns_mhdp_bridge_hpd_enable(bridge);
@@ -2012,6 +2040,16 @@ static enum drm_connector_status
 cdns_mhdp_bridge_detect(struct drm_bridge *bridge, struct drm_connector *connector)
 {
 	struct cdns_mhdp_device *mhdp = bridge_to_mhdp(bridge);
+	bool hw_ready;
+
+	if (mhdp->no_hpd) {
+		spin_lock(&mhdp->start_lock);
+		hw_ready = mhdp->hw_state == MHDP_HW_READY;
+		spin_unlock(&mhdp->start_lock);
+
+		if (hw_ready)
+			cdns_mhdp_update_link_status(mhdp);
+	}
 
 	return cdns_mhdp_detect(mhdp);
 }
@@ -2100,7 +2138,29 @@ 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) {
+		u8 rev;
+
+		/*
+		 * Use a side-effect-free capability register for presence
+		 * detection. A successful AUX read means a sink is present;
+		 * we do not need link training status registers here.
+		 */
+		ret = drm_dp_dpcd_read(&mhdp->aux, DP_DPCD_REV, &rev, 1);
+		mhdp->plugged = (ret == 1);
+		ret = mhdp->plugged ? 0 : -EIO;
+		hpd_pulse = false;
+
+		/*
+		 * If the monitor is still connected and the link is already
+		 * up, there is nothing to do. Avoid falling through to
+		 * cdns_mhdp_sst_enable() on every poll cycle.
+		 */
+		if (mhdp->plugged && old_plugged && mhdp->link_up)
+			goto out;
+	} else {
+		mhdp->plugged = cdns_mhdp_detect_hpd(mhdp, &hpd_pulse);
+	}
 
 	if (!mhdp->plugged) {
 		cdns_mhdp_link_down(mhdp);
@@ -2288,6 +2348,8 @@ 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 +2422,9 @@ 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