usb: gadget: configfs: allow setting function instance's name

USB function's configfs config group is created in a generic way in
usb/gadget/configfs.c:function_make(), which in turn delegates actual
allocation and setup of the USB function instance to a particular
implementation, e.g. in f_acm.c. The said implementation does its job
in a parameter-less function e.g. acm_alloc_instance(), which results
in creating an unnamed config group, whose name is set later in
function_make(). function_make() creates the name by parsing a string
of the form:

<function name>.<instance name>

which comes from userspace as a parameter to mkdir invocation.

Up to now only <function name> has been used, while <instance name>
has been ignored. This patch adds a set_inst_name() operation to
struct usb_function_instance which allows passing the <instance name>
from function_make() so that it is not ignored. It is entirely up to the
implementor of set_inst_name() what to do with the <instance name>.

In a typical case, the struct usb_function_instance is embedded in a
larger struct which is retrieved in set_inst_name() with container_of(),
and the larger struct contains a field to store the <instance name>.

Signed-off-by: Andrzej Pietrasiewicz <[e-mail]>
Signed-off-by: Kyungmin Park <[e-mail]>
Acked-by: Michal Nazarewicz <[e-mail]>
Signed-off-by: Felipe Balbi <[e-mail]>

drivers/usb/gadget/configfs.c

···
 		usb_put_function_instance(fi);
 		return ERR_PTR(ret);
 	}
+	if (fi->set_inst_name) {
+		ret = fi->set_inst_name(fi, instance_name);
+		if (ret) {
+			usb_put_function_instance(fi);
+			return ERR_PTR(ret);
+		}
+	}
 
 	gi = container_of(group, struct gadget_info, functions_group);
 

include/linux/usb/composite.h

···
 	struct config_group group;
 	struct list_head cfs_list;
 	struct usb_function_driver *fd;
+	int (*set_inst_name)(struct usb_function_instance *inst,
+			      const char *name);
 	void (*free_func_inst)(struct usb_function_instance *inst);
 };