usb: resume child device when port is powered on

Unconditionally wake up the child device when the power session is
recovered.

This addresses the following scenarios:

1/ The device may need a reset on power-session loss, without this
change port power-on recovery exposes khubd to scenarios that
usb_port_resume() is set to handle. Prior to port power control the
only time a power session would be lost is during dpm_suspend of the
hub. In that scenario usb_port_resume() is guaranteed to be called
prior to khubd running for that port. With this change we wakeup the
child device as soon as possible (prior to khubd running again for this
port).

Although khubd has facilities to wake a child device it will only do
so if the portstatus / portchange indicates a suspend state. In the
case of port power control we are not coming from a hub-port-suspend
state. This implementation simply uses pm_request_resume() to wake the
device and relies on the port_dev->status_lock to prevent any collisions
between khubd and usb_port_resume().

2/ This mechanism rate limits port power toggling. The minimum port
power on/off period is now gated by the child device suspend/resume
latency. Empirically this mitigates devices downgrading their connection
on perceived instability of the host connection. This ratelimiting is
really only relevant to port power control testing, but it is a nice
side effect of closing the above race. Namely, the race of khubd for
the given port running while a usb_port_resume() event is pending.

3/ Going forward we are finding that power-session recovery requires
warm-resets (http://marc.info/?t=138659232900003&r=1&w=2). This
mechanism allows for warm-resets to be requested at the same point in
the resume path for hub dpm_suspend power session losses, or port
rpm_suspend power session losses.

4/ If the device *was* disconnected the only time we'll know for sure is
after a failed resume, so it's necessary for usb_port_runtime_resume()
to expedite a usb_port_resume() to clean up the removed device. The
reasoning for this is "least surprise" for the user. Turning on a port
means that hotplug detection is again enabled for the port, it is
surprising that devices that were removed while the port was off are not
disconnected until they are attempted to be used. As a user "why would
I try to use a device I removed from the system?"

1, 2, and 4 are not a problem in the system dpm_resume() case because,
although the power-session is lost, khubd is frozen until after device
resume. For the rpm_resume() case pm_request_resume() is used to
request re-validation of the device, and if it happens to collide with a
khubd run we rely on the port_dev->status_lock to synchronize those
operations.

Besides testing, the primary scenario where this mechanism is expected
to be triggered is when the user changes the port power policy
(control/pm_qos_no_poweroff, or power/control). Each time power is
enabled want to revalidate the child device, where the revalidation is
handled by usb_port_resume().

Given that this arranges for port_dev->child to be de-referenced in
usb_port_runtime_resume() we need to make sure not to collide with
usb_disconnect() that frees the usb_device. To this end we hold the
port active with the "child_usage" reference across the disconnect
event. Subsequently, the need to access hub->child_usage_bits lead to
the creation of hub_disconnect_children() to remove any ambiguity of
which "hub" is being acted on in usb_disconnect() (prompted-by sharp
eyes from Alan).

Cc: Rafael J. Wysocki <[e-mail]>
Acked-by: Alan Stern <[e-mail]>
Signed-off-by: Dan Williams <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/core/hub.c

···
 		hcd->driver->free_dev(hcd, udev);
 }
 
+static void hub_disconnect_children(struct usb_device *udev)
+{
+	struct usb_hub *hub = usb_hub_to_struct_hub(udev);
+	int i;
+
+	/* Free up all the children before we remove this device */
+	for (i = 0; i < udev->maxchild; i++) {
+		if (hub->ports[i]->child)
+			usb_disconnect(&hub->ports[i]->child);
+	}
+}
+
 /**
  * usb_disconnect - disconnect a device (usbcore-internal)
  * @pdev: pointer to device being disconnected
···
  */
 void usb_disconnect(struct usb_device **pdev)
 {
-	struct usb_device	*udev = *pdev;
-	struct usb_hub		*hub = usb_hub_to_struct_hub(udev);
-	int			i;
+	struct usb_port *port_dev = NULL;
+	struct usb_device *udev = *pdev;
+	struct usb_hub *hub;
+	int port1;
 
 	/* mark the device as inactive, so any further urb submissions for
 	 * this device (and any of its children) will fail immediately.
···
 
 	usb_lock_device(udev);
 
-	/* Free up all the children before we remove this device */
-	for (i = 0; i < udev->maxchild; i++) {
-		if (hub->ports[i]->child)
-			usb_disconnect(&hub->ports[i]->child);
-	}
+	hub_disconnect_children(udev);
 
 	/* deallocate hcd/hardware state ... nuking all pending urbs and
 	 * cleaning up all state associated with the current configuration
···
 	usb_hcd_synchronize_unlinks(udev);
 
 	if (udev->parent) {
-		int port1 = udev->portnum;
-		struct usb_hub *hub = usb_hub_to_struct_hub(udev->parent);
-		struct usb_port	*port_dev = hub->ports[port1 - 1];
+		port1 = udev->portnum;
+		hub = usb_hub_to_struct_hub(udev->parent);
+		port_dev = hub->ports[port1 - 1];
 
 		sysfs_remove_link(&udev->dev.kobj, "port");
 		sysfs_remove_link(&port_dev->dev.kobj, "device");
 
-		if (test_and_clear_bit(port1, hub->child_usage_bits))
-			pm_runtime_put(&port_dev->dev);
+		/*
+		 * As usb_port_runtime_resume() de-references udev, make
+		 * sure no resumes occur during removal
+		 */
+		if (!test_and_set_bit(port1, hub->child_usage_bits))
+			pm_runtime_get_sync(&port_dev->dev);
 	}
 
 	usb_remove_ep_devs(&udev->ep0);
···
 	spin_lock_irq(&device_state_lock);
 	*pdev = NULL;
 	spin_unlock_irq(&device_state_lock);
+
+	if (port_dev && test_and_clear_bit(port1, hub->child_usage_bits))
+		pm_runtime_put(&port_dev->dev);
 
 	hub_free_dev(udev);
 

drivers/usb/core/port.c

···
 	struct usb_device *hdev = to_usb_device(dev->parent->parent);
 	struct usb_interface *intf = to_usb_interface(dev->parent);
 	struct usb_hub *hub = usb_hub_to_struct_hub(hdev);
+	struct usb_device *udev = port_dev->child;
 	struct usb_port *peer = port_dev->peer;
 	int port1 = port_dev->portnum;
 	int retval;
···
 	usb_autopm_get_interface(intf);
 	retval = usb_hub_set_port_power(hdev, hub, port1, true);
 	msleep(hub_power_on_good_delay(hub));
-	if (port_dev->child && !retval) {
+	if (udev && !retval) {
 		/*
 		 * Attempt to wait for usb hub port to be reconnected in order
 		 * to make the resume procedure successful.  The device may have
···
 			dev_dbg(&port_dev->dev, "can't get reconnection after setting port  power on, status %d\n",
 					retval);
 		retval = 0;
+
+		/* Force the child awake to revalidate after the power loss. */
+		if (!test_and_set_bit(port1, hub->child_usage_bits)) {
+			pm_runtime_get_noresume(&port_dev->dev);
+			pm_request_resume(&udev->dev);
+		}
 	}
 
 	usb_autopm_put_interface(intf);