USB: add power/persist device attribute

This patch (as920) adds an extra level of protection to the
USB-Persist facility.  Now it will apply by default only to hubs; for
all other devices the user must enable it explicitly by setting the
power/persist device attribute.

The disconnect_all_children() routine in hub.c has been removed and
its code placed inline.  This is the way it was originally as part of
hub_pre_reset(); the revised usage in hub_reset_resume() is
sufficiently different that the code can no longer be shared.
Likewise, mark_children_for_reset() is now inline as part of
hub_reset_resume().  The end result looks much cleaner than before.

The sysfs interface is updated to add the new attribute file, and
there are corresponding documentation updates.

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

6 files changed, 149 insertions, 69 deletions

diff --git a/Documentation/ABI/testing/sysfs-bus-usb b/Documentation/ABI/testing/sysfs-bus-usb
index f9937add033d..9734577d1711 100644
--- a/Documentation/ABI/testing/sysfs-bus-usb
+++ b/Documentation/ABI/testing/sysfs-bus-usb
@@ -39,3 +39,16 @@ Description:
                 If you want to suspend a device immediately but leave it
                 free to wake up in response to I/O requests, you should
                 write "0" to power/autosuspend.
+
+What:           /sys/bus/usb/devices/.../power/persist
+Date:           May 2007
+KernelVersion:  2.6.23
+Contact:        Alan Stern <stern@rowland.harvard.edu>
+Description:
+                If CONFIG_USB_PERSIST is set, then each USB device directory
+                will contain a file named power/persist.  The file holds a
+                boolean value (0 or 1) indicating whether or not the
+                "USB-Persist" facility is enabled for the device.  Since the
+                facility is inherently dangerous, it is disabled by default
+                for all devices except hubs.  For more information, see
+                Documentation/usb/persist.txt.
diff --git a/Documentation/usb/persist.txt b/Documentation/usb/persist.txt
index 6dcd5f884795..df54d645cbb5 100644
--- a/Documentation/usb/persist.txt
+++ b/Documentation/usb/persist.txt
@@ -2,7 +2,7 @@
 
                    Alan Stern <stern@rowland.harvard.edu>
 
-                 September 2, 2006 (Updated March 27, 2007)
+                 September 2, 2006 (Updated May 29, 2007)
 
 
         What is the problem?
@@ -52,9 +52,9 @@ you can convince the BIOS supplier to fix the problem (lots of luck!).
 
 On many systems the USB host controllers will get reset after a
 suspend-to-RAM.  On almost all systems, no suspend current is
-available during suspend-to-disk (also known as swsusp).  You can
-check the kernel log after resuming to see if either of these has
-happened; look for lines saying "root hub lost power or was reset".
+available during hibernation (also known as swsusp or suspend-to-disk).
+You can check the kernel log after resuming to see if either of these
+has happened; look for lines saying "root hub lost power or was reset".
 
 In practice, people are forced to unmount any filesystems on a USB
 device before suspending.  If the root filesystem is on a USB device,
@@ -71,15 +71,16 @@ structures are allowed to persist across a power-session disruption.
 It works like this.  If the kernel sees that a USB host controller is
 not in the expected state during resume (i.e., if the controller was
 reset or otherwise had lost power) then it applies a persistence check
-to each of the USB devices below that controller.  It doesn't try to
-resume the device; that can't work once the power session is gone.
-Instead it issues a USB port reset and then re-enumerates the device.
-(This is exactly the same thing that happens whenever a USB device is
-reset.)  If the re-enumeration shows that the device now attached to
-that port has the same descriptors as before, including the Vendor and
-Product IDs, then the kernel continues to use the same device
-structure.  In effect, the kernel treats the device as though it had
-merely been reset instead of unplugged.
+to each of the USB devices below that controller for which the
+"persist" attribute is set.  It doesn't try to resume the device; that
+can't work once the power session is gone.  Instead it issues a USB
+port reset and then re-enumerates the device.  (This is exactly the
+same thing that happens whenever a USB device is reset.)  If the
+re-enumeration shows that the device now attached to that port has the
+same descriptors as before, including the Vendor and Product IDs, then
+the kernel continues to use the same device structure.  In effect, the
+kernel treats the device as though it had merely been reset instead of
+unplugged.
 
 If no device is now attached to the port, or if the descriptors are
 different from what the kernel remembers, then the treatment is what
@@ -91,6 +92,17 @@ The end result is that the USB device remains available and usable.
 Filesystem mounts and memory mappings are unaffected, and the world is
 now a good and happy place.
 
+Note that even when CONFIG_USB_PERSIST is set, the "persist" feature
+will be applied only to those devices for which it is enabled.  You
+can enable the feature by doing (as root):
+
+        echo 1 >/sys/bus/usb/devices/.../power/persist
+
+where the "..." should be filled in the with the device's ID.  Disable
+the feature by writing 0 instead of 1.  For hubs the feature is
+automatically and permanently enabled, so you only have to worry about
+setting it for devices where it really matters.
+
 
         Is this the best solution?
 
diff --git a/drivers/usb/core/Kconfig b/drivers/usb/core/Kconfig
index 5113ef4cb7f6..97b09f282705 100644
--- a/drivers/usb/core/Kconfig
+++ b/drivers/usb/core/Kconfig
@@ -91,12 +91,15 @@ config USB_PERSIST
         depends on USB && PM && EXPERIMENTAL
         default n
         help
-          If you say Y here, USB device data structures will remain
+
+          If you say Y here and enable the "power/persist" attribute
+          for a USB device, the device's data structures will remain
           persistent across system suspend, even if the USB bus loses
-          power.  (This includes software-suspend, also known as swsusp,
-          or suspend-to-disk.)  The devices will reappear as if by magic
-          when the system wakes up, with no need to unmount USB filesystems,
-          rmmod host-controller drivers, or do anything else.
+          power.  (This includes hibernation, also known as swsusp or
+          suspend-to-disk.)  The devices will reappear as if by magic
+          when the system wakes up, with no need to unmount USB
+          filesystems, rmmod host-controller drivers, or do anything
+          else.
 
                 WARNING: This option can be dangerous!
 
diff --git a/drivers/usb/core/hub.c b/drivers/usb/core/hub.c
index c4cdb69a6e9e..50e79010401c 100644
--- a/drivers/usb/core/hub.c
+++ b/drivers/usb/core/hub.c
@@ -596,27 +596,18 @@ static void hub_port_logical_disconnect(struct usb_hub *hub, int port1)
         kick_khubd(hub);
 }
 
-static void disconnect_all_children(struct usb_hub *hub, int logical)
-{
-        struct usb_device *hdev = hub->hdev;
-        int port1;
-
-        for (port1 = 1; port1 <= hdev->maxchild; ++port1) {
-                if (hdev->children[port1-1]) {
-                        if (logical)
-                                hub_port_logical_disconnect(hub, port1);
-                        else
-                                usb_disconnect(&hdev->children[port1-1]);
-                }
-        }
-}
-
 /* caller has locked the hub device */
 static int hub_pre_reset(struct usb_interface *intf)
 {
         struct usb_hub *hub = usb_get_intfdata(intf);
+        struct usb_device *hdev = hub->hdev;
+        int i;
 
-        disconnect_all_children(hub, 0);
+        /* Disconnect all the children */
+        for (i = 0; i < hdev->maxchild; ++i) {
+                if (hdev->children[i])
+                        usb_disconnect(&hdev->children[i]);
+        }
         hub_quiesce(hub);
         return 0;
 }
@@ -1872,50 +1863,39 @@ static int hub_resume(struct usb_interface *intf)
         return 0;
 }
 
-#ifdef  CONFIG_USB_PERSIST
-
-/* For "persistent-device" resets we must mark the child devices for reset
- * and turn off a possible connect-change status (so khubd won't disconnect
- * them later).
- */
-static void mark_children_for_reset_resume(struct usb_hub *hub)
+static int hub_reset_resume(struct usb_interface *intf)
 {
+        struct usb_hub *hub = usb_get_intfdata(intf);
         struct usb_device *hdev = hub->hdev;
         int port1;
 
+        hub_power_on(hub);
+
         for (port1 = 1; port1 <= hdev->maxchild; ++port1) {
                 struct usb_device *child = hdev->children[port1-1];
 
                 if (child) {
-                        child->reset_resume = 1;
-                        clear_port_feature(hdev, port1,
-                                        USB_PORT_FEAT_C_CONNECTION);
+
+                        /* For "USB_PERSIST"-enabled children we must
+                         * mark the child device for reset-resume and
+                         * turn off the connect-change status to prevent
+                         * khubd from disconnecting it later.
+                         */
+                        if (USB_PERSIST && child->persist_enabled) {
+                                child->reset_resume = 1;
+                                clear_port_feature(hdev, port1,
+                                                USB_PORT_FEAT_C_CONNECTION);
+
+                        /* Otherwise we must disconnect the child,
+                         * but as we may not lock the child device here
+                         * we have to do a "logical" disconnect.
+                         */
+                        } else {
+                                hub_port_logical_disconnect(hub, port1);
+                        }
                 }
         }
-}
-
-#else
-
-static inline void mark_children_for_reset_resume(struct usb_hub *hub)
-{ }
-
-#endif  /* CONFIG_USB_PERSIST */
-
-static int hub_reset_resume(struct usb_interface *intf)
-{
-        struct usb_hub *hub = usb_get_intfdata(intf);
 
-        hub_power_on(hub);
-        if (USB_PERSIST)
-                mark_children_for_reset_resume(hub);
-        else {
-                /* Reset-resume doesn't call pre_reset, so we have to
-                 * disconnect the children here.  But we may not lock
-                 * the child devices, so we have to do a "logical"
-                 * disconnect.
-                 */
-                disconnect_all_children(hub, 1);
-        }
         hub_activate(hub);
         return 0;
 }
diff --git a/drivers/usb/core/sysfs.c b/drivers/usb/core/sysfs.c
index be37c863fdfb..5dfe31bc32ba 100644
--- a/drivers/usb/core/sysfs.c
+++ b/drivers/usb/core/sysfs.c
@@ -169,6 +169,73 @@ show_quirks(struct device *dev, struct device_attribute *attr, char *buf)
 }
 static DEVICE_ATTR(quirks, S_IRUGO, show_quirks, NULL);
 
+
+#if defined(CONFIG_USB_PERSIST) || defined(CONFIG_USB_SUSPEND)
+static const char power_group[] = "power";
+#endif
+
+#ifdef  CONFIG_USB_PERSIST
+
+static ssize_t
+show_persist(struct device *dev, struct device_attribute *attr, char *buf)
+{
+        struct usb_device *udev = to_usb_device(dev);
+
+        return sprintf(buf, "%d\n", udev->persist_enabled);
+}
+
+static ssize_t
+set_persist(struct device *dev, struct device_attribute *attr,
+                const char *buf, size_t count)
+{
+        struct usb_device *udev = to_usb_device(dev);
+        int value;
+
+        /* Hubs are always enabled for USB_PERSIST */
+        if (udev->descriptor.bDeviceClass == USB_CLASS_HUB)
+                return -EPERM;
+
+        if (sscanf(buf, "%d", &value) != 1)
+                return -EINVAL;
+        usb_pm_lock(udev);
+        udev->persist_enabled = !!value;
+        usb_pm_unlock(udev);
+        return count;
+}
+
+static DEVICE_ATTR(persist, S_IRUGO | S_IWUSR, show_persist, set_persist);
+
+static int add_persist_attributes(struct device *dev)
+{
+        int rc = 0;
+
+        if (is_usb_device(dev)) {
+                struct usb_device *udev = to_usb_device(dev);
+
+                /* Hubs are automatically enabled for USB_PERSIST */
+                if (udev->descriptor.bDeviceClass == USB_CLASS_HUB)
+                        udev->persist_enabled = 1;
+                rc = sysfs_add_file_to_group(&dev->kobj,
+                                &dev_attr_persist.attr,
+                                power_group);
+        }
+        return rc;
+}
+
+static void remove_persist_attributes(struct device *dev)
+{
+        sysfs_remove_file_from_group(&dev->kobj,
+                        &dev_attr_persist.attr,
+                        power_group);
+}
+
+#else
+
+#define add_persist_attributes(dev)     0
+#define remove_persist_attributes(dev)  do {} while (0)
+
+#endif  /* CONFIG_USB_PERSIST */
+
 #ifdef  CONFIG_USB_SUSPEND
 
 static ssize_t
@@ -276,8 +343,6 @@ set_level(struct device *dev, struct device_attribute *attr,
 
 static DEVICE_ATTR(level, S_IRUGO | S_IWUSR, show_level, set_level);
 
-static char power_group[] = "power";
-
 static int add_power_attributes(struct device *dev)
 {
         int rc = 0;
@@ -311,6 +376,7 @@ static void remove_power_attributes(struct device *dev)
 
 #endif  /* CONFIG_USB_SUSPEND */
 
+
 /* Descriptor fields */
 #define usb_descriptor_attr_le16(field, format_string)                  \
 static ssize_t                                                          \
@@ -384,6 +450,10 @@ int usb_create_sysfs_dev_files(struct usb_device *udev)
         if (retval)
                 return retval;
 
+        retval = add_persist_attributes(dev);
+        if (retval)
+                goto error;
+
         retval = add_power_attributes(dev);
         if (retval)
                 goto error;
@@ -421,6 +491,7 @@ void usb_remove_sysfs_dev_files(struct usb_device *udev)
         device_remove_file(dev, &dev_attr_product);
         device_remove_file(dev, &dev_attr_serial);
         remove_power_attributes(dev);
+        remove_persist_attributes(dev);
         sysfs_remove_group(&dev->kobj, &dev_attr_grp);
 }
 
diff --git a/include/linux/usb.h b/include/linux/usb.h
index bde8c65e2bfc..efce9a4c511c 100644
--- a/include/linux/usb.h
+++ b/include/linux/usb.h
@@ -404,6 +404,7 @@ struct usb_device {
         unsigned auto_pm:1;             /* autosuspend/resume in progress */
         unsigned do_remote_wakeup:1;    /* remote wakeup should be enabled */
         unsigned reset_resume:1;        /* needs reset instead of resume */
+        unsigned persist_enabled:1;     /* USB_PERSIST enabled for this dev */
         unsigned autosuspend_disabled:1; /* autosuspend and autoresume */
         unsigned autoresume_disabled:1;  /*  disabled by the user */
 #endif