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>

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.

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?
 

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!
 

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;
 }

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);
 }
 

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