USB: Make it possible to "subclass" usb_device_driver

The kernel currenly has only 2 usb_device_drivers, one generic one, one
that completely replaces the generic one to make USB devices usable over
a network.

Use the newly exported generic driver functions when a driver declares
to want them run, in addition to its own code. This makes it possible to
write drivers that extend the generic USB driver.

Note that this patch is not enough for another driver to automatically
get selected.

Signed-off-by: Bastien Nocera <hadess@hadess.net>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Link: https://lore.kernel.org/r/20191016093933.693-3-hadess@hadess.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/core/driver.c

@@ -261,9 +261,16 @@ static int usb_probe_device(struct device *dev)
 	 */
 	if (!udriver->supports_autosuspend)
 		error = usb_autoresume_device(udev);
+	if (error)
+		return error;
 
-	if (!error)
-		error = udriver->probe(udev);
+	if (udriver->generic_subclass)
+		error = usb_generic_driver_probe(udev);
+	if (error)
+		return error;
+
+	error = udriver->probe(udev);
+	/* TODO: fallback to generic driver in case of error */
 	return error;
 }
 
@@ -273,7 +280,10 @@ static int usb_unbind_device(struct device *dev)
 	struct usb_device *udev = to_usb_device(dev);
 	struct usb_device_driver *udriver = to_usb_device_driver(dev->driver);
 
-	udriver->disconnect(udev);
+	if (udriver->disconnect)
+		udriver->disconnect(udev);
+	if (udriver->generic_subclass)
+		usb_generic_driver_disconnect(udev);
 	if (!udriver->supports_autosuspend)
 		usb_autosuspend_device(udev);
 	return 0;
@@ -1149,7 +1159,10 @@ static int usb_suspend_device(struct usb_device *udev, pm_message_t msg)
 		udev->do_remote_wakeup = 0;
 		udriver = &usb_generic_driver;
 	}
-	status = udriver->suspend(udev, msg);
+	if (udriver->suspend)
+		status = udriver->suspend(udev, msg);
+	if (status == 0 && udriver->generic_subclass)
+		status = usb_generic_driver_suspend(udev, msg);
 
  done:
 	dev_vdbg(&udev->dev, "%s: status %d\n", __func__, status);
@@ -1181,7 +1194,10 @@ static int usb_resume_device(struct usb_device *udev, pm_message_t msg)
 		udev->reset_resume = 1;
 
 	udriver = to_usb_device_driver(udev->dev.driver);
-	status = udriver->resume(udev, msg);
+	if (udriver->generic_subclass)
+		status = usb_generic_driver_resume(udev, msg);
+	if (status == 0 && udriver->resume)
+		status = udriver->resume(udev, msg);
 
  done:
 	dev_vdbg(&udev->dev, "%s: status %d\n", __func__, status);

include/linux/usb.h

@@ -1228,6 +1228,9 @@ struct usb_driver {
  * @drvwrap: Driver-model core structure wrapper.
  * @supports_autosuspend: if set to 0, the USB core will not allow autosuspend
  *	for devices bound to this driver.
+ * @generic_subclass: if set to 1, the generic USB driver's probe, disconnect,
+ *	resume and suspend functions will be called in addition to the driver's
+ *	own, so this part of the setup does not need to be replicated.
  *
  * USB drivers must provide all the fields listed above except drvwrap.
  */
@@ -1242,6 +1245,7 @@ struct usb_device_driver {
 	const struct attribute_group **dev_groups;
 	struct usbdrv_wrap drvwrap;
 	unsigned int supports_autosuspend:1;
+	unsigned int generic_subclass:1;
 };
 #define	to_usb_device_driver(d) container_of(d, struct usb_device_driver, \
 		drvwrap.driver)