usb: add usb_set_intfdata() documentation

USB drivers do not need to call usb_set_intfdata(intf, NULL) in their
usb_driver::disconnect callback because the core already does it in [1].

However, this fact is widely unknown, c.f.:

$ git grep "usb_set_intfdata(.*NULL)" | wc -l
215

Especially, setting the interface to NULL before all action completed
can result in a NULL pointer dereference. Not calling
usb_set_intfdata() at all in disconnect() is the safest method.

Add documentation to usb_set_intfdata() to clarify this point.

Also remove the call in usb-skeletion's disconnect() not to confuse
the new comers.

[1] function usb_unbind_interface() from drivers/usb/core/driver.c
Link: https://elixir.bootlin.com/linux/v6.0/source/drivers/usb/core/driver.c#L497

Signed-off-by: Vincent Mailhol <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/usb-skeleton.c

···
 	int minor = interface->minor;
 
 	dev = usb_get_intfdata(interface);
-	usb_set_intfdata(interface, NULL);
 
 	/* give back our minor */
 	usb_deregister_dev(interface, &skel_class);

include/linux/usb.h

···
 	return dev_get_drvdata(&intf->dev);
 }
 
+/**
+ * usb_set_intfdata() - associate driver-specific data with the interface
+ * @intf: the usb interface
+ * @data: pointer to the device priv structure or %NULL
+ *
+ * Drivers should use this function in their probe() to associate their
+ * driver-specific data with the usb interface.
+ *
+ * When disconnecting, the core will take care of setting @intf back to %NULL,
+ * so no actions are needed on the driver side. The interface should not be set
+ * to %NULL before all actions completed (e.g. no outsanding URB remaining).
+ */
 static inline void usb_set_intfdata(struct usb_interface *intf, void *data)
 {
 	dev_set_drvdata(&intf->dev, data);