intel_th: gth: Handle host mode correctly

When detecting host debugger mode either though a module option or via
the scratchpad register, do not export any configuration or capture
related attributes to userspace and refuse attempts by the output drivers
to configure output ports.

This way, GTH can still act as a hub and ensure that the other components
that rely on its presence continue to function properly, namely the
source devices.
Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>

drivers/hwtracing/intel_th/gth.c

@@ -564,6 +564,9 @@ static int intel_th_gth_assign(struct intel_th_device *thdev,
 	struct gth_device *gth = dev_get_drvdata(&thdev->dev);
 	int i, id;
 
+	if (thdev->host_mode)
+		return -EBUSY;
+
 	if (othdev->type != INTEL_TH_OUTPUT)
 		return -EINVAL;
 
@@ -600,6 +603,9 @@ static void intel_th_gth_unassign(struct intel_th_device *thdev,
 	struct gth_device *gth = dev_get_drvdata(&thdev->dev);
 	int port = othdev->output.port;
 
+	if (thdev->host_mode)
+		return;
+
 	spin_lock(&gth->gth_lock);
 	othdev->output.port = -1;
 	othdev->output.active = false;
@@ -654,10 +660,25 @@ static int intel_th_gth_probe(struct intel_th_device *thdev)
 	gth->base = base;
 	spin_lock_init(&gth->gth_lock);
 
+	/*
+	 * Host mode can be signalled via SW means or via SCRPD_DEBUGGER_IN_USE
+	 * bit. Either way, don't reset HW in this case, and don't export any
+	 * capture configuration attributes. Also, refuse to assign output
+	 * drivers to ports, see intel_th_gth_assign().
+	 */
+	if (thdev->host_mode)
+		goto done;
+
 	ret = intel_th_gth_reset(gth);
-	if (ret)
+	if (ret) {
+		if (ret != -EBUSY)
 			return ret;
 
+		thdev->host_mode = true;
+
+		goto done;
+	}
+
 	for (i = 0; i < TH_CONFIGURABLE_MASTERS + 1; i++)
 		gth->master[i] = -1;
 
@@ -677,6 +698,7 @@ static int intel_th_gth_probe(struct intel_th_device *thdev)
 		return -ENOMEM;
 	}
 
+done:
 	dev_set_drvdata(dev, gth);
 
 	return 0;