USB: debounce before unregistering

This patch (as1080) makes a significant change to the way khubd
handles port connect-change and enable-change events.  Both types of
event are now debounced, and the debouncing is carried out _before_ an
existing usb_device is unregistered, instead of afterward.

This means that drivers will have to deal with longer runs of errors
when a device is unplugged, but they are supposed to be prepared for
that in any case.

The advantage is that when an enable-change occurs (caused for example
by electromagnetic interference), the debouncing period will provide
time for the cause of the problem to die away.  A simple port reset
(added in a forthcoming patch) will then allow us to recover from the
fault.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

1 files changed, 19 insertions, 12 deletions

diff --git a/drivers/usb/core/hub.c b/drivers/usb/core/hub.c
index d14da2123eb5..d741b9457427 100644
--- a/drivers/usb/core/hub.c
+++ b/drivers/usb/core/hub.c
@@ -2673,9 +2673,10 @@ 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);
-        u16 wHubCharacteristics = le16_to_cpu(hub->descriptor->wHubCharacteristics);
+        unsigned wHubCharacteristics =
+                        le16_to_cpu(hub->descriptor->wHubCharacteristics);
         int status, i;
- 
+
         dev_dbg (hub_dev,
                 "port %d, status %04x, change %04x, %s\n",
                 port1, portstatus, portchange, portspeed (portstatus));
@@ -2684,30 +2685,36 @@ static void hub_port_connect_change(struct usb_hub *hub, int port1,
                 set_port_led(hub, port1, HUB_LED_AUTO);
                 hub->indicator[port1-1] = INDICATOR_AUTO;
         }
- 
-        /* Disconnect any existing devices under this port */
-        if (hdev->children[port1-1])
-                usb_disconnect(&hdev->children[port1-1]);
-        clear_bit(port1, hub->change_bits);
 
 #ifdef  CONFIG_USB_OTG
         /* during HNP, don't repeat the debounce */
         if (hdev->bus->is_b_host)
-                portchange &= ~USB_PORT_STAT_C_CONNECTION;
+                portchange &= ~(USB_PORT_STAT_C_CONNECTION |
+                                USB_PORT_STAT_C_ENABLE);
 #endif
 
-        if (portchange & USB_PORT_STAT_C_CONNECTION) {
+        /* Try to use the debounce delay for protection against
+         * port-enable changes caused, for example, by EMI.
+         */
+        if (portchange & (USB_PORT_STAT_C_CONNECTION |
+                                USB_PORT_STAT_C_ENABLE)) {
                 status = hub_port_debounce(hub, port1);
                 if (status < 0) {
                         if (printk_ratelimit())
                                 dev_err (hub_dev, "connect-debounce failed, "
                                                 "port %d disabled\n", port1);
-                        goto done;
+                        portstatus &= ~USB_PORT_STAT_CONNECTION;
+                } else {
+                        portstatus = status;
                 }
-                portstatus = status;
         }
 
-        /* Return now if nothing is connected */
+        /* Disconnect any existing devices under this port */
+        if (hdev->children[port1-1])
+                usb_disconnect(&hdev->children[port1-1]);
+        clear_bit(port1, hub->change_bits);
+
+        /* Return now if debouncing failed or nothing is connected */
         if (!(portstatus & USB_PORT_STAT_CONNECTION)) {
 
                 /* maybe switch power back on (e.g. root hub was reset) */