usb: gadget: uvc: Allow definition of XUs in configfs

The UVC gadget at present has no support for extension units. Add the
infrastructure to uvc_configfs.c that allows users to create XUs via
configfs. These will be stored in a new child of uvcg_control_grp_type
with the name "extensions".

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Daniel Scally <dan.scally@ideasonboard.com>
Link: https://lore.kernel.org/r/20230206161802.892954-4-dan.scally@ideasonboard.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

@@ -113,6 +113,34 @@ Description:	Default processing unit descriptors
 		bUnitID		a non-zero id of this unit
 		===============	========================================
 
+What:		/config/usb-gadget/gadget/functions/uvc.name/control/extensions
+Date:		Nov 2022
+KernelVersion:	6.1
+Description:	Extension unit descriptors
+
+What:		/config/usb-gadget/gadget/functions/uvc.name/control/extensions/name
+Date:		Nov 2022
+KernelVersion:	6.1
+Description:	Extension Unit (XU) Descriptor
+
+		bLength, bUnitID and iExtension are read-only. All others are
+		read-write.
+
+		=================	========================================
+		bLength			size of the descriptor in bytes
+		bUnitID			non-zero ID of this unit
+		guidExtensionCode	Vendor-specific code identifying the XU
+		bNumControls		number of controls in this XU
+		bNrInPins		number of input pins for this unit
+		baSourceID		list of the IDs of the units or terminals
+					to which this XU is connected
+		bControlSize		size of the bmControls field in bytes
+		bmControls		list of bitmaps detailing which vendor
+					specific controls are supported
+		iExtension		index of a string descriptor that describes
+					this extension unit
+		=================	========================================
+
 What:		/config/usb-gadget/gadget/functions/uvc.name/control/header
 Date:		Dec 2014
 KernelVersion:	4.0

drivers/usb/gadget/function/f_uvc.c

@@ -865,6 +865,13 @@ static struct usb_function_instance *uvc_alloc_inst(void)
 	od->bSourceID			= 2;
 	od->iTerminal			= 0;
 
+	/*
+	 * With the ability to add XUs to the UVC function graph, we need to be
+	 * able to allocate unique unit IDs to them. The IDs are 1-based, with
+	 * the CT, PU and OT above consuming the first 3.
+	 */
+	opts->last_unit_id		= 3;
+
 	/* Prepare fs control class descriptors for configfs-based gadgets */
 	ctl_cls = opts->uvc_fs_control_cls;
 	ctl_cls[0] = NULL;	/* assigned elsewhere by configfs */
@@ -885,6 +892,8 @@ static struct usb_function_instance *uvc_alloc_inst(void)
 	opts->ss_control =
 		(const struct uvc_descriptor_header * const *)ctl_cls;
 
+	INIT_LIST_HEAD(&opts->extension_units);
+
 	opts->streaming_interval = 1;
 	opts->streaming_maxpacket = 1024;
 	snprintf(opts->function_name, sizeof(opts->function_name), "UVC Camera");

drivers/usb/gadget/function/u_uvc.h

@@ -28,6 +28,7 @@ struct f_uvc_opts {
 	unsigned int					control_interface;
 	unsigned int					streaming_interface;
 	char						function_name[32];
+	unsigned int					last_unit_id;
 
 	bool						enable_interrupt_ep;
 
@@ -65,6 +66,12 @@ struct f_uvc_opts {
 	struct uvc_descriptor_header			*uvc_fs_control_cls[5];
 	struct uvc_descriptor_header			*uvc_ss_control_cls[5];
 
+	/*
+	 * Control descriptors for extension units. There could be any number
+	 * of these, including none at all.
+	 */
+	struct list_head				extension_units;
+
 	/*
 	 * Streaming descriptors for full-speed, high-speed and super-speed.
 	 * Used by configfs only, must not be touched by legacy gadgets. The