staging: comedi: split out USB support into new module

Setting the `CONFIG_COMEDI_USB_DRIVERS` kernel configuration option
makes the main "comedi" module depend on the "usbcore" module.  But
perhaps the machine has no USB controllers (or they have been disabled),
in which case the "usbcore" module may have been pulled in
unnecessarily.  Only a few low-level Comedi drivers require USB support.
The Comedi USB support functions are all in "comedi_usb.c".  Turn it
into a separate module so we don't have to pull in the "usbcore" and
"usb_common" modules unnecessarily.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/Kconfig

@@ -1155,11 +1155,14 @@ config COMEDI_QUATECH_DAQP_CS
 endif # COMEDI_PCMCIA_DRIVERS
 
 menuconfig COMEDI_USB_DRIVERS
-	bool "Comedi USB drivers"
+	tristate "Comedi USB drivers"
 	depends on USB
 	---help---
 	  Enable support for comedi USB drivers.
 
+	  To compile this support as a module, choose M here: the module will
+	  be called comedi_usb.
+
 if COMEDI_USB_DRIVERS
 
 config COMEDI_DT9812

drivers/staging/comedi/Makefile

@@ -3,11 +3,11 @@ ccflags-$(CONFIG_COMEDI_DEBUG)		:= -DDEBUG
 comedi-y				:= comedi_fops.o range.o drivers.o \
 					   comedi_buf.o
 comedi-$(CONFIG_COMEDI_PCI_DRIVERS)	+= comedi_pci.o
-comedi-$(CONFIG_COMEDI_USB_DRIVERS)	+= comedi_usb.o
 comedi-$(CONFIG_PROC_FS)		+= proc.o
 comedi-$(CONFIG_COMPAT)			+= comedi_compat32.o
 
 obj-$(CONFIG_COMEDI_PCMCIA_DRIVERS)	+= comedi_pcmcia.o
+obj-$(CONFIG_COMEDI_USB_DRIVERS)	+= comedi_usb.o
 
 obj-$(CONFIG_COMEDI)			+= comedi.o
 

drivers/staging/comedi/comedi_usb.c

@@ -16,6 +16,7 @@
  * GNU General Public License for more details.
  */
 
+#include <linux/module.h>
 #include <linux/usb.h>
 
 #include "comedidev.h"
@@ -114,3 +115,18 @@ void comedi_usb_driver_unregister(struct comedi_driver *comedi_driver,
 	comedi_driver_unregister(comedi_driver);
 }
 EXPORT_SYMBOL_GPL(comedi_usb_driver_unregister);
+
+static int __init comedi_usb_init(void)
+{
+	return 0;
+}
+module_init(comedi_usb_init);
+
+static void __exit comedi_usb_exit(void)
+{
+}
+module_exit(comedi_usb_exit);
+
+MODULE_AUTHOR("http://www.comedi.org");
+MODULE_DESCRIPTION("Comedi USB interface module");
+MODULE_LICENSE("GPL");