intel_th: Support Host Debugger mode of operation

This patch adds a 'host_mode' module option to enable host-driven
operational mode in the driver. In this mode, the driver does not
perform trace configuration or enable trace capture, but still
provides all the means necessary for software trace sources to
write their data to the Trace Hub. This means that the debug host
takes care of all the configuration and enabling and we do not
interfere.

Signed-off-by: Alexander Shishkin <[e-mail]>

drivers/hwtracing/intel_th/core.c

···
 #include "intel_th.h"
 #include "debug.h"
 
+static bool host_mode __read_mostly;
+module_param(host_mode, bool, 0444);
+
 static DEFINE_IDA(intel_th_ida);
 
 static int intel_th_match(struct device *dev, struct device_driver *driver)
···
 {
 	struct resource res[3];
 	unsigned int req = 0;
-	int i, err;
+	int src, dst, err;
 
 	/* create devices for each intel_th_subdevice */
-	for (i = 0; i < ARRAY_SIZE(intel_th_subdevices); i++) {
-		struct intel_th_subdevice *subdev = &intel_th_subdevices[i];
+	for (src = 0, dst = 0; src < ARRAY_SIZE(intel_th_subdevices); src++) {
+		const struct intel_th_subdevice *subdev =
+			&intel_th_subdevices[src];
 		struct intel_th_device *thdev;
 		int r;
+
+		/* only allow SOURCE and SWITCH devices in host mode */
+		if (host_mode && subdev->type == INTEL_TH_OUTPUT)
+			continue;
 
 		thdev = intel_th_device_alloc(th, subdev->type, subdev->name,
 					      subdev->id);
···
 		}
 
 		if (subdev->type == INTEL_TH_OUTPUT) {
-			thdev->dev.devt = MKDEV(th->major, i);
+			thdev->dev.devt = MKDEV(th->major, dst);
 			thdev->output.type = subdev->otype;
 			thdev->output.port = -1;
 			thdev->output.scratchpad = subdev->scrpd;
+		} else if (subdev->type == INTEL_TH_SWITCH) {
+			thdev->host_mode = host_mode;
 		}
 
 		err = device_add(&thdev->dev);
···
 				req++;
 		}
 
-		th->thdev[i] = thdev;
+		th->thdev[dst++] = thdev;
 	}
 
 	return 0;
 
 kill_subdevs:
-	for (i-- ; i >= 0; i--)
-		intel_th_device_remove(th->thdev[i]);
+	for (; dst >= 0; dst--)
+		intel_th_device_remove(th->thdev[dst]);
 
 	return err;
 }
···
 
 	intel_th_request_hub_module_flush(th);
 	for (i = 0; i < TH_SUBDEVICE_MAX; i++)
-		if (th->thdev[i] != th->hub)
+		if (th->thdev[i] && th->thdev[i] != th->hub)
 			intel_th_device_remove(th->thdev[i]);
 
 	intel_th_device_remove(th->hub);

drivers/hwtracing/intel_th/intel_th.h

···
  * @num_resources:	number of resources in @resource array
  * @type:		INTEL_TH_{SOURCE,OUTPUT,SWITCH}
  * @id:			device instance or -1
+ * @host_mode:		Intel TH is controlled by an external debug host
  * @output:		output descriptor for INTEL_TH_OUTPUT devices
  * @name:		device name to match the driver
  */
···
 	unsigned int	num_resources;
 	unsigned int	type;
 	int		id;
+
+	/* INTEL_TH_SWITCH specific */
+	bool			host_mode;
 
 	/* INTEL_TH_OUTPUT specific */
 	struct intel_th_output	output;