usb: gadget: uvc: configfs: Add interface number attributes

The video control and video streaming interface numbers are needed in
the UVC gadget userspace stack to reply to UVC requests. They are
hardcoded to fixed values at the moment, preventing configurations with
multiple functions.

To fix this, make them dynamically discoverable by userspace through
read-only configfs attributes in <function>/control/bInterfaceNumber and
<function>/streaming/bInterfaceNumber respectively.
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

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

@@ -12,6 +12,10 @@ Date:		Dec 2014
 KernelVersion:	4.0
 Description:	Control descriptors
 
+		All attributes read only:
+		bInterfaceNumber	- USB interface number for this
+					  streaming interface
+
 What:		/config/usb-gadget/gadget/functions/uvc.name/control/class
 Date:		Dec 2014
 KernelVersion:	4.0
@@ -109,6 +113,10 @@ Date:		Dec 2014
 KernelVersion:	4.0
 Description:	Streaming descriptors
 
+		All attributes read only:
+		bInterfaceNumber	- USB interface number for this
+					  streaming interface
+
 What:		/config/usb-gadget/gadget/functions/uvc.name/streaming/class
 Date:		Dec 2014
 KernelVersion:	4.0

drivers/usb/gadget/function/f_uvc.c

@@ -699,12 +699,14 @@ uvc_function_bind(struct usb_configuration *c, struct usb_function *f)
 	uvc_iad.bFirstInterface = ret;
 	uvc_control_intf.bInterfaceNumber = ret;
 	uvc->control_intf = ret;
+	opts->control_interface = ret;
 
 	if ((ret = usb_interface_id(c, f)) < 0)
 		goto error;
 	uvc_streaming_intf_alt0.bInterfaceNumber = ret;
 	uvc_streaming_intf_alt1.bInterfaceNumber = ret;
 	uvc->streaming_intf = ret;
+	opts->streaming_interface = ret;
 
 	/* Copy descriptors */
 	f->fs_descriptors = uvc_copy_descriptors(uvc, USB_SPEED_FULL);

drivers/usb/gadget/function/u_uvc.h

@@ -25,6 +25,9 @@ struct f_uvc_opts {
 	unsigned int					streaming_maxpacket;
 	unsigned int					streaming_maxburst;
 
+	unsigned int					control_interface;
+	unsigned int					streaming_interface;
+
 	/*
 	 * Control descriptors array pointers for full-/high-speed and
 	 * super-speed. They point by default to the uvc_fs_control_cls and

drivers/usb/gadget/function/uvc_configfs.c

@@ -713,9 +713,40 @@ static const struct uvcg_config_group_type uvcg_control_class_grp_type = {
  * control
  */
 
+static ssize_t uvcg_default_control_b_interface_number_show(
+	struct config_item *item, char *page)
+{
+	struct config_group *group = to_config_group(item);
+	struct mutex *su_mutex = &group->cg_subsys->su_mutex;
+	struct config_item *opts_item;
+	struct f_uvc_opts *opts;
+	int result = 0;
+
+	mutex_lock(su_mutex); /* for navigating configfs hierarchy */
+
+	opts_item = item->ci_parent;
+	opts = to_f_uvc_opts(opts_item);
+
+	mutex_lock(&opts->lock);
+	result += sprintf(page, "%u\n", opts->control_interface);
+	mutex_unlock(&opts->lock);
+
+	mutex_unlock(su_mutex);
+
+	return result;
+}
+
+UVC_ATTR_RO(uvcg_default_control_, b_interface_number, bInterfaceNumber);
+
+static struct configfs_attribute *uvcg_default_control_attrs[] = {
+	&uvcg_default_control_attr_b_interface_number,
+	NULL,
+};
+
 static const struct uvcg_config_group_type uvcg_control_grp_type = {
 	.type = {
 		.ct_item_ops	= &uvcg_config_item_ops,
+		.ct_attrs	= uvcg_default_control_attrs,
 		.ct_owner	= THIS_MODULE,
 	},
 	.name = "control",
@@ -2259,9 +2290,40 @@ static const struct uvcg_config_group_type uvcg_streaming_class_grp_type = {
  * streaming
  */
 
+static ssize_t uvcg_default_streaming_b_interface_number_show(
+	struct config_item *item, char *page)
+{
+	struct config_group *group = to_config_group(item);
+	struct mutex *su_mutex = &group->cg_subsys->su_mutex;
+	struct config_item *opts_item;
+	struct f_uvc_opts *opts;
+	int result = 0;
+
+	mutex_lock(su_mutex); /* for navigating configfs hierarchy */
+
+	opts_item = item->ci_parent;
+	opts = to_f_uvc_opts(opts_item);
+
+	mutex_lock(&opts->lock);
+	result += sprintf(page, "%u\n", opts->streaming_interface);
+	mutex_unlock(&opts->lock);
+
+	mutex_unlock(su_mutex);
+
+	return result;
+}
+
+UVC_ATTR_RO(uvcg_default_streaming_, b_interface_number, bInterfaceNumber);
+
+static struct configfs_attribute *uvcg_default_streaming_attrs[] = {
+	&uvcg_default_streaming_attr_b_interface_number,
+	NULL,
+};
+
 static const struct uvcg_config_group_type uvcg_streaming_grp_type = {
 	.type = {
 		.ct_item_ops	= &uvcg_config_item_ops,
+		.ct_attrs	= uvcg_default_streaming_attrs,
 		.ct_owner	= THIS_MODULE,
 	},
 	.name = "streaming",