[PATCH] usb: hub: Wait for connection to be reestablished after port reset

[Guenter Roeck <linux@roeck-us.net>]

On a system with a defective USB device connected to an USB hub,
an endless sequence of port connect events was observed. The sequence
of events as observed is as follows:

- Port reports connected event (port status=USB_PORT_STAT_CONNECTION).
- Event handler debounces port and resets it by calling hub_port_reset().
- hub_port_reset() calls hub_port_wait_reset() to wait for the reset
  to complete.
- The reset completes, but USB_PORT_STAT_CONNECTION is not immediately
  set in the port status register.
- hub_port_wait_reset() returns -ENOTCONN.
- Port initialization sequence is aborted.
- A few milliseconds later, the port again reports a connected event,
  and the sequence repeats.

This continues either forever or, randomly, stops if the connection
is already re-established when the port status is read. It results in
a high rate of udev events. This in turn destabilizes userspace since
the above sequence holds the device mutex pretty much continuously
and prevents userspace from actually reading the device status.

To prevent the problem from happening, let's wait for the connection
to be re-established after a port reset. If the device was actually
disconnected, the code will still return an error, but it will do so
only after the long reset timeout.

Cc: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
---
 drivers/usb/core/hub.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/drivers/usb/core/hub.c b/drivers/usb/core/hub.c
index cbb146736f57..9bcb649e0e8c 100644
--- a/drivers/usb/core/hub.c
+++ b/drivers/usb/core/hub.c
@@ -2731,8 +2731,15 @@ static int hub_port_wait_reset(struct usb_hub *hub, int port1,
 		if (ret < 0)
 			return ret;
 
-		/* The port state is unknown until the reset completes. */
-		if (!(portstatus & USB_PORT_STAT_RESET))
+		/*
+		 * The port state is unknown until the reset completes.
+		 *
+		 * On top of that, some chips may require additional time
+		 * to re-establish a connection after the reset is complete,
+		 * so also wait for the connection to be re-established.
+		 */
+		if (!(portstatus & USB_PORT_STAT_RESET) &&
+		    (portstatus & USB_PORT_STAT_CONNECTION))
 			break;
 
 		/* switch to the long delay after two short delay failures */
-- 
2.5.0