PM / sleep: prohibit devices probing during suspend/hibernation

It is unsafe [1] if probing of devices will happen during suspend or
hibernation and system behavior will be unpredictable in this case.
So, let's prohibit device's probing in dpm_prepare() and defer their
probing instead. The normal behavior will be restored in
dpm_complete().

This patch introduces new DD core APIs:
 device_block_probing()
   It will disable probing of devices and defer their probes instead.
 device_unblock_probing()
   It will restore normal behavior and trigger re-probing of deferred
   devices.

[1] https://lkml.org/lkml/2015/9/11/554

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/base/base.h

@@ -131,6 +131,8 @@ extern void device_remove_groups(struct device *dev,
 extern char *make_class_name(const char *name, struct kobject *kobj);
 
 extern int devres_release_all(struct device *dev);
+extern void device_block_probing(void);
+extern void device_unblock_probing(void);
 
 /* /sys/devices directory */
 extern struct kset *devices_kset;

drivers/base/dd.c

@@ -54,6 +54,13 @@ static LIST_HEAD(deferred_probe_active_list);
 static struct workqueue_struct *deferred_wq;
 static atomic_t deferred_trigger_count = ATOMIC_INIT(0);
 
+/*
+ * In some cases, like suspend to RAM or hibernation, It might be reasonable
+ * to prohibit probing of devices as it could be unsafe.
+ * Once defer_all_probes is true all drivers probes will be forcibly deferred.
+ */
+static bool defer_all_probes;
+
 /*
  * deferred_probe_work_func() - Retry probing devices in the active list.
  */
@@ -171,6 +178,30 @@ static void driver_deferred_probe_trigger(void)
 	queue_work(deferred_wq, &deferred_probe_work);
 }
 
+/**
+ * device_block_probing() - Block/defere device's probes
+ *
+ *	It will disable probing of devices and defer their probes instead.
+ */
+void device_block_probing(void)
+{
+	defer_all_probes = true;
+	/* sync with probes to avoid races. */
+	wait_for_device_probe();
+}
+
+/**
+ * device_unblock_probing() - Unblock/enable device's probes
+ *
+ *	It will restore normal behavior and trigger re-probing of deferred
+ * devices.
+ */
+void device_unblock_probing(void)
+{
+	defer_all_probes = false;
+	driver_deferred_probe_trigger();
+}
+
 /**
  * deferred_probe_initcall() - Enable probing of deferred devices
  *
@@ -277,9 +308,20 @@ static DECLARE_WAIT_QUEUE_HEAD(probe_waitqueue);
 
 static int really_probe(struct device *dev, struct device_driver *drv)
 {
-	int ret = 0;
+	int ret = -EPROBE_DEFER;
 	int local_trigger_count = atomic_read(&deferred_trigger_count);
 
+	if (defer_all_probes) {
+		/*
+		 * Value of defer_all_probes can be set only by
+		 * device_defer_all_probes_enable() which, in turn, will call
+		 * wait_for_device_probe() right after that to avoid any races.
+		 */
+		dev_dbg(dev, "Driver %s force probe deferral\n", drv->name);
+		driver_deferred_probe_add(dev);
+		return ret;
+	}
+
 	atomic_inc(&probe_count);
 	pr_debug("bus: '%s': %s: probing driver %s with device %s\n",
 		 drv->bus->name, __func__, drv->name, dev_name(dev));
@@ -393,6 +435,10 @@ int driver_probe_done(void)
  */
 void wait_for_device_probe(void)
 {
+	/* wait for the deferred probe workqueue to finish */
+	if (driver_deferred_probe_enable)
+		flush_workqueue(deferred_wq);
+
 	/* wait for the known devices to complete their probing */
 	wait_event(probe_waitqueue, atomic_read(&probe_count) == 0);
 	async_synchronize_full();

drivers/base/power/main.c

@@ -963,6 +963,9 @@ void dpm_complete(pm_message_t state)
 	}
 	list_splice(&list, &dpm_list);
 	mutex_unlock(&dpm_list_mtx);
+
+	/* Allow device probing and trigger re-probing of deferred devices */
+	device_unblock_probing();
 	trace_suspend_resume(TPS("dpm_complete"), state.event, false);
 }
 
@@ -1624,6 +1627,20 @@ int dpm_prepare(pm_message_t state)
 	trace_suspend_resume(TPS("dpm_prepare"), state.event, true);
 	might_sleep();
 
+	/*
+	 * Give a chance for the known devices to complete their probes, before
+	 * disable probing of devices. This sync point is important at least
+	 * at boot time + hibernation restore.
+	 */
+	wait_for_device_probe();
+	/*
+	 * It is unsafe if probing of devices will happen during suspend or
+	 * hibernation and system behavior will be unpredictable in this case.
+	 * So, let's prohibit device's probing here and defer their probes
+	 * instead. The normal behavior will be restored in dpm_complete().
+	 */
+	device_block_probing();
+
 	mutex_lock(&dpm_list_mtx);
 	while (!list_empty(&dpm_list)) {
 		struct device *dev = to_device(dpm_list.next);