USB: leave LPM alone if possible when binding/unbinding interface drivers

When a USB driver is bound to an interface (either through probing or
by claiming it) or is unbound from an interface, the USB core always
disables Link Power Management during the transition and then
re-enables it afterward. The reason is because the driver might want
to prevent hub-initiated link power transitions, in which case the HCD
would have to recalculate the various LPM parameters. This
recalculation takes place when LPM is re-enabled and the new
parameters are sent to the device and its parent hub.

However, if the driver does not want to prevent hub-initiated link
power transitions then none of this work is necessary. The parameters
don't need to be recalculated, and LPM doesn't need to be disabled and
re-enabled.

It turns out that disabling and enabling LPM can be time-consuming,
enough so that it interferes with user programs that want to claim and
release interfaces rapidly via usbfs. Since the usbfs kernel driver
doesn't set the disable_hub_initiated_lpm flag, we can speed things up
and get the user programs to work by leaving LPM alone whenever the
flag isn't set.

And while we're improving the way disable_hub_initiated_lpm gets used,
let's also fix its kerneldoc.

Signed-off-by: Alan Stern <[e-mail]>
Tested-by: Matthew Giassa <[e-mail]>
CC: Mathias Nyman <[e-mail]>
CC: <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/core/driver.c

···
 	struct usb_device *udev = interface_to_usbdev(intf);
 	const struct usb_device_id *id;
 	int error = -ENODEV;
-	int lpm_disable_error;
+	int lpm_disable_error = -ENODEV;
 
 	dev_dbg(dev, "%s\n", __func__);
 
···
 	 * setting during probe, that should also be fine.  usb_set_interface()
 	 * will attempt to disable LPM, and fail if it can't disable it.
 	 */
-	lpm_disable_error = usb_unlocked_disable_lpm(udev);
-	if (lpm_disable_error && driver->disable_hub_initiated_lpm) {
-		dev_err(&intf->dev, "%s Failed to disable LPM for driver %s\n.",
-				__func__, driver->name);
-		error = lpm_disable_error;
-		goto err;
+	if (driver->disable_hub_initiated_lpm) {
+		lpm_disable_error = usb_unlocked_disable_lpm(udev);
+		if (lpm_disable_error) {
+			dev_err(&intf->dev, "%s Failed to disable LPM for driver %s\n.",
+					__func__, driver->name);
+			error = lpm_disable_error;
+			goto err;
+		}
 	}
 
 	/* Carry out a deferred switch to altsetting 0 */
···
 	struct usb_interface *intf = to_usb_interface(dev);
 	struct usb_host_endpoint *ep, **eps = NULL;
 	struct usb_device *udev;
-	int i, j, error, r, lpm_disable_error;
+	int i, j, error, r;
+	int lpm_disable_error = -ENODEV;
 
 	intf->condition = USB_INTERFACE_UNBINDING;
 
···
 	udev = interface_to_usbdev(intf);
 	error = usb_autoresume_device(udev);
 
-	/* Hub-initiated LPM policy may change, so attempt to disable LPM until
+	/* If hub-initiated LPM policy may change, attempt to disable LPM until
 	 * the driver is unbound.  If LPM isn't disabled, that's fine because it
 	 * wouldn't be enabled unless all the bound interfaces supported
 	 * hub-initiated LPM.
 	 */
-	lpm_disable_error = usb_unlocked_disable_lpm(udev);
+	if (driver->disable_hub_initiated_lpm)
+		lpm_disable_error = usb_unlocked_disable_lpm(udev);
 
 	/*
 	 * Terminate all URBs for this interface unless the driver
···
 	struct device *dev;
 	struct usb_device *udev;
 	int retval = 0;
-	int lpm_disable_error;
+	int lpm_disable_error = -ENODEV;
 
 	if (!iface)
 		return -ENODEV;
···
 
 	iface->condition = USB_INTERFACE_BOUND;
 
-	/* Disable LPM until this driver is bound. */
-	lpm_disable_error = usb_unlocked_disable_lpm(udev);
-	if (lpm_disable_error && driver->disable_hub_initiated_lpm) {
-		dev_err(&iface->dev, "%s Failed to disable LPM for driver %s\n.",
-				__func__, driver->name);
-		return -ENOMEM;
+	/* See the comment about disabling LPM in usb_probe_interface(). */
+	if (driver->disable_hub_initiated_lpm) {
+		lpm_disable_error = usb_unlocked_disable_lpm(udev);
+		if (lpm_disable_error) {
+			dev_err(&iface->dev, "%s Failed to disable LPM for driver %s\n.",
+					__func__, driver->name);
+			return -ENOMEM;
+		}
 	}
 
 	/* Claimed interfaces are initially inactive (suspended) and

include/linux/usb.h

···
  *	for interfaces bound to this driver.
  * @soft_unbind: if set to 1, the USB core will not kill URBs and disable
  *	endpoints before calling the driver's disconnect method.
- * @disable_hub_initiated_lpm: if set to 0, the USB core will not allow hubs
+ * @disable_hub_initiated_lpm: if set to 1, the USB core will not allow hubs
  *	to initiate lower power link state transitions when an idle timeout
  *	occurs.  Device-initiated USB 3.0 link PM will still be allowed.
  *