usb: disable port power control if not supported in wHubCharacteristics

A hub indicates whether it supports per-port power control via the
wHubCharacteristics field in its descriptor.  If it is not supported
a hub will still emulate ClearPortPower(PORT_POWER) requests by
stopping the link state machine.  However, since this does not save
power do not bother suspending.

This also consolidates support checks into a
hub_is_port_power_switchable() helper.
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/core/hub.c

@@ -818,8 +818,6 @@ static unsigned hub_power_on(struct usb_hub *hub, bool do_delay)
 	int port1;
 	unsigned pgood_delay = hub->descriptor->bPwrOn2PwrGood * 2;
 	unsigned delay;
-	u16 wHubCharacteristics =
-			le16_to_cpu(hub->descriptor->wHubCharacteristics);
 
 	/* Enable power on each port.  Some hubs have reserved values
 	 * of LPSM (> 2) in their descriptors, even though they are
@@ -827,7 +825,7 @@ static unsigned hub_power_on(struct usb_hub *hub, bool do_delay)
 	 * but only emulate it.  In all cases, the ports won't work
 	 * unless we send these messages to the hub.
 	 */
-	if ((wHubCharacteristics & HUB_CHAR_LPSM) < 2)
+	if (hub_is_port_power_switchable(hub))
 		dev_dbg(hub->intfdev, "enabling power on all ports\n");
 	else
 		dev_dbg(hub->intfdev, "trying to enable port power on "
@@ -4417,8 +4415,6 @@ static void hub_port_connect_change(struct usb_hub *hub, int port1,
 	struct usb_device *hdev = hub->hdev;
 	struct device *hub_dev = hub->intfdev;
 	struct usb_hcd *hcd = bus_to_hcd(hdev->bus);
-	unsigned wHubCharacteristics =
-			le16_to_cpu(hub->descriptor->wHubCharacteristics);
 	struct usb_device *udev;
 	int status, i;
 	unsigned unit_load;
@@ -4503,7 +4499,7 @@ static void hub_port_connect_change(struct usb_hub *hub, int port1,
 			test_bit(port1, hub->removed_bits)) {
 
 		/* maybe switch power back on (e.g. root hub was reset) */
-		if ((wHubCharacteristics & HUB_CHAR_LPSM) < 2
+		if (hub_is_port_power_switchable(hub)
 				&& !port_is_power_on(hub, portstatus))
 			set_port_feature(hdev, port1, USB_PORT_FEAT_POWER);
 

drivers/usb/core/hub.h

@@ -112,6 +112,16 @@ extern int hub_port_debounce(struct usb_hub *hub, int port1,
 extern int usb_clear_port_feature(struct usb_device *hdev,
 		int port1, int feature);
 
+static inline bool hub_is_port_power_switchable(struct usb_hub *hub)
+{
+	__le16 hcs;
+
+	if (!hub)
+		return false;
+	hcs = hub->descriptor->wHubCharacteristics;
+	return (le16_to_cpu(hcs) & HUB_CHAR_LPSM) < HUB_CHAR_NO_LPSM;
+}
+
 static inline int hub_port_debounce_be_connected(struct usb_hub *hub,
 		int port1)
 {

drivers/usb/core/port.c

@@ -177,12 +177,15 @@ int usb_hub_create_port_device(struct usb_hub *hub, int port1)
 
 	pm_runtime_set_active(&port_dev->dev);
 
-	/* It would be dangerous if user space couldn't
-	 * prevent usb device from being powered off. So don't
-	 * enable port runtime pm if failed to expose port's pm qos.
+	/*
+	 * Do not enable port runtime pm if the hub does not support
+	 * power switching.  Also, userspace must have final say of
+	 * whether a port is permitted to power-off.  Do not enable
+	 * runtime pm if we fail to expose pm_qos_no_power_off.
 	 */
-	if (!dev_pm_qos_expose_flags(&port_dev->dev,
-			PM_QOS_FLAG_NO_POWER_OFF))
+	if (hub_is_port_power_switchable(hub)
+			&& dev_pm_qos_expose_flags(&port_dev->dev,
+			PM_QOS_FLAG_NO_POWER_OFF) == 0)
 		pm_runtime_enable(&port_dev->dev);
 
 	device_enable_async_suspend(&port_dev->dev);