driver core: Add deferred_probe attribute to devices in sysfs

It is sometimes useful to know that a device is on the deferred probe
list rather than, say, not having a driver available.  Expose this
information to user-space.
Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/ABI/testing/sysfs-devices-deferred_probe

+What:		/sys/devices/.../deferred_probe
+Date:		August 2016
+Contact:	Ben Hutchings <ben.hutchings@codethink.co.uk>
+Description:
+		The /sys/devices/.../deferred_probe attribute is
+		present for all devices.  If a driver detects during
+		probing a device that a related device is not yet
+		ready, it may defer probing of the first device.  The
+		kernel will retry probing the first device after any
+		other device is successfully probed.  This attribute
+		reads as 1 if probing of this device is currently
+		deferred, or 0 otherwise.

drivers/base/base.h

@@ -141,6 +141,8 @@ extern void device_unblock_probing(void);
 extern struct kset *devices_kset;
 extern void devices_kset_move_last(struct device *dev);
 
+extern struct device_attribute dev_attr_deferred_probe;
+
 #if defined(CONFIG_MODULES) && defined(CONFIG_SYSFS)
 extern void module_add_driver(struct module *mod, struct device_driver *drv);
 extern void module_remove_driver(struct device_driver *drv);

drivers/base/core.c

@@ -1060,8 +1060,14 @@ static int device_add_attrs(struct device *dev)
 			goto err_remove_dev_groups;
 	}
 
+	error = device_create_file(dev, &dev_attr_deferred_probe);
+	if (error)
+		goto err_remove_online;
+
 	return 0;
 
+ err_remove_online:
+	device_remove_file(dev, &dev_attr_online);
  err_remove_dev_groups:
 	device_remove_groups(dev, dev->groups);
  err_remove_type_groups:
@@ -1079,6 +1085,7 @@ static void device_remove_attrs(struct device *dev)
 	struct class *class = dev->class;
 	const struct device_type *type = dev->type;
 
+	device_remove_file(dev, &dev_attr_deferred_probe);
 	device_remove_file(dev, &dev_attr_online);
 	device_remove_groups(dev, dev->groups);
 

drivers/base/dd.c

@@ -53,6 +53,19 @@ static LIST_HEAD(deferred_probe_pending_list);
 static LIST_HEAD(deferred_probe_active_list);
 static atomic_t deferred_trigger_count = ATOMIC_INIT(0);
 
+static ssize_t deferred_probe_show(struct device *dev,
+				   struct device_attribute *attr, char *buf)
+{
+	bool value;
+
+	mutex_lock(&deferred_probe_mutex);
+	value = !list_empty(&dev->p->deferred_probe);
+	mutex_unlock(&deferred_probe_mutex);
+
+	return sprintf(buf, "%d\n", value);
+}
+DEVICE_ATTR_RO(deferred_probe);
+
 /*
  * In some cases, like suspend to RAM or hibernation, It might be reasonable
  * to prohibit probing of devices as it could be unsafe.