usb: gadget: uvc: Expose configuration name through video node

When utilising multiple instantiations of a UVC gadget on a composite
device, there is no clear method to link a particular configuration to
its respective video node.

Provide a means for identifying the correct video node by exposing the
name of the function configuration through sysfs.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

Documentation/ABI/testing/configfs-usb-gadget-uvc

@@ -263,3 +263,8 @@ Description:	Specific streaming header descriptors
 					is connected
 		bmInfo			- capabilities of this video streaming
 					interface
+
+What:		/sys/class/udc/udc.name/device/gadget/video4linux/video.name/function_name
+Date:		May 2018
+KernelVersion:	4.19
+Description:	UVC configfs function instance name

drivers/usb/gadget/function/f_uvc.c

@@ -413,10 +413,21 @@ uvc_function_disconnect(struct uvc_device *uvc)
  * USB probe and disconnect
  */
 
+static ssize_t function_name_show(struct device *dev,
+				  struct device_attribute *attr, char *buf)
+{
+	struct uvc_device *uvc = dev_get_drvdata(dev);
+
+	return sprintf(buf, "%s\n", uvc->func.fi->group.cg_item.ci_name);
+}
+
+static DEVICE_ATTR_RO(function_name);
+
 static int
 uvc_register_video(struct uvc_device *uvc)
 {
 	struct usb_composite_dev *cdev = uvc->func.config->cdev;
+	int ret;
 
 	/* TODO reference counting. */
 	uvc->vdev.v4l2_dev = &uvc->v4l2_dev;
@@ -429,7 +440,17 @@ uvc_register_video(struct uvc_device *uvc)
 
 	video_set_drvdata(&uvc->vdev, uvc);
 
-	return video_register_device(&uvc->vdev, VFL_TYPE_GRABBER, -1);
+	ret = video_register_device(&uvc->vdev, VFL_TYPE_GRABBER, -1);
+	if (ret < 0)
+		return ret;
+
+	ret = device_create_file(&uvc->vdev.dev, &dev_attr_function_name);
+	if (ret < 0) {
+		video_unregister_device(&uvc->vdev);
+		return ret;
+	}
+
+	return 0;
 }
 
 #define UVC_COPY_DESCRIPTOR(mem, dst, desc) \
@@ -867,6 +888,7 @@ static void uvc_unbind(struct usb_configuration *c, struct usb_function *f)
 
 	INFO(cdev, "%s\n", __func__);
 
+	device_remove_file(&uvc->vdev.dev, &dev_attr_function_name);
 	video_unregister_device(&uvc->vdev);
 	v4l2_device_unregister(&uvc->v4l2_dev);