usb: gadget: add some infracture to register/unregister functions

This patch provides an infrastructure to register & unregister a USB
function. This allows to turn a function into a module and avoid the
'#include "f_.*.c"' magic and we get a clear API / cut between the bare
gadget and its functions.
The concept is simple:
Each function defines the DECLARE_USB_FUNCTION_INIT macro whith an unique
name of the function and two allocation functions.
- one to create an "instance". The instance holds the current configuration
  set. In case there are two usb_configudations with one function there will
  be one instance and two usb_functions
- one to create an "function" from the instance.

The name of the instance is used to automaticaly load the module if it the
instance is not yet available.
The usb_function callbacks are slightly modified and extended:
- usb_get_function()
  creates a struct usb_function inclunding all pointers (bind,
  unbind,…). It uses the "instance" to map its configuration. So we can
  have _two_ struct usb_function, one for each usb_configuration.
- ->unbind()
  Since the struct usb_function was not allocated in ->bind() it should
  not kfree()d here. This function should only reverse what happens in
  ->bind() that is request cleanup and the cleanup of allocated
  descriptors.
- ->free_func()
  a simple kfree() of the struct usb_function

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>

include/linux/usb/composite.h

@@ -77,6 +77,8 @@ struct usb_configuration;
  *	in interface or class descriptors; endpoints; I/O buffers; and so on.
  * @unbind: Reverses @bind; called as a side effect of unregistering the
  *	driver which added this function.
+ * @free_func: free the struct usb_function.
+ * @mod: (internal) points to the module that created this structure.
  * @set_alt: (REQUIRED) Reconfigures altsettings; function drivers may
  *	initialize usb_ep.driver data at this time (when it is used).
  *	Note that setting an interface to its current altsetting resets
@@ -116,6 +118,7 @@ struct usb_configuration;
  * two or more distinct instances within the same configuration, providing
  * several independent logical data links to a USB host.
  */
+
 struct usb_function {
 	const char			*name;
 	struct usb_gadget_strings	**strings;
@@ -136,6 +139,8 @@ struct usb_function {
 					struct usb_function *);
 	void			(*unbind)(struct usb_configuration *,
 					struct usb_function *);
+	void			(*free_func)(struct usb_function *f);
+	struct module		*mod;
 
 	/* runtime state management */
 	int			(*set_alt)(struct usb_function *,
@@ -432,6 +437,53 @@ static inline u16 get_default_bcdDevice(void)
 	return bcdDevice;
 }
 
+struct usb_function_driver {
+	const char *name;
+	struct module *mod;
+	struct list_head list;
+	struct usb_function_instance *(*alloc_inst)(void);
+	struct usb_function *(*alloc_func)(struct usb_function_instance *inst);
+};
+
+struct usb_function_instance {
+	struct usb_function_driver *fd;
+	void (*free_func_inst)(struct usb_function_instance *inst);
+};
+
+void usb_function_unregister(struct usb_function_driver *f);
+int usb_function_register(struct usb_function_driver *newf);
+void usb_put_function_instance(struct usb_function_instance *fi);
+void usb_put_function(struct usb_function *f);
+struct usb_function_instance *usb_get_function_instance(const char *name);
+struct usb_function *usb_get_function(struct usb_function_instance *fi);
+
+struct usb_configuration *usb_get_config(struct usb_composite_dev *cdev,
+		int val);
+int usb_add_config_only(struct usb_composite_dev *cdev,
+		struct usb_configuration *config);
+
+#define DECLARE_USB_FUNCTION(_name, _inst_alloc, _func_alloc)		\
+	static struct usb_function_driver _name ## usb_func = {		\
+		.name = __stringify(_name),				\
+		.mod  = THIS_MODULE,					\
+		.alloc_inst = _inst_alloc,				\
+		.alloc_func = _func_alloc,				\
+	};								\
+	MODULE_ALIAS("usbfunc:"__stringify(_name));
+
+#define DECLARE_USB_FUNCTION_INIT(_name, _inst_alloc, _func_alloc)	\
+	DECLARE_USB_FUNCTION(_name, _inst_alloc, _func_alloc)		\
+	static int __init _name ## mod_init(void)			\
+	{								\
+		return usb_function_register(&_name ## usb_func);	\
+	}								\
+	static void __exit _name ## mod_exit(void)			\
+	{								\
+		usb_function_unregister(&_name ## usb_func);		\
+	}								\
+	module_init(_name ## mod_init);					\
+	module_exit(_name ## mod_exit)
+
 /* messaging utils */
 #define DBG(d, fmt, args...) \
 	dev_dbg(&(d)->gadget->dev , fmt , ## args)