PM / Runtime: Defer resuming of the device in pm_runtime_force_resume()

When the pm_runtime_force_suspend|resume() helpers were invented, we still
had CONFIG_PM_RUNTIME and CONFIG_PM_SLEEP as separate Kconfig options.

To make sure these helpers worked for all combinations and without
introducing too much of complexity, the device was always resumed in
pm_runtime_force_resume().

More precisely, when CONFIG_PM_SLEEP was set and CONFIG_PM_RUNTIME was
unset, we needed to resume the device as the subsystem/driver couldn't
rely on using runtime PM to do it.

As the CONFIG_PM_RUNTIME option was merged into CONFIG_PM a while ago, it
removed this combination, of using CONFIG_PM_SLEEP without the earlier
CONFIG_PM_RUNTIME.

For this reason we can now rely on the subsystem/driver to use runtime PM
to resume the device, instead of forcing that to be done in all cases. In
other words, let's defer the runtime resume to a later point when it's
actually needed.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/base/power/runtime.c

@@ -1489,6 +1489,16 @@ int pm_runtime_force_suspend(struct device *dev)
 	if (ret)
 		goto err;
 
+	/*
+	 * Increase the runtime PM usage count for the device's parent, in case
+	 * when we find the device being used when system suspend was invoked.
+	 * This informs pm_runtime_force_resume() to resume the parent
+	 * immediately, which is needed to be able to resume its children,
+	 * when not deferring the resume to be managed via runtime PM.
+	 */
+	if (dev->parent && atomic_read(&dev->power.usage_count) > 1)
+		pm_runtime_get_noresume(dev->parent);
+
 	pm_runtime_set_suspended(dev);
 	return 0;
 err:
@@ -1498,16 +1508,20 @@ err:
 EXPORT_SYMBOL_GPL(pm_runtime_force_suspend);
 
 /**
- * pm_runtime_force_resume - Force a device into resume state.
+ * pm_runtime_force_resume - Force a device into resume state if needed.
  * @dev: Device to resume.
  *
  * Prior invoking this function we expect the user to have brought the device
  * into low power state by a call to pm_runtime_force_suspend(). Here we reverse
- * those actions and brings the device into full power. We update the runtime PM
- * status and re-enables runtime PM.
+ * those actions and brings the device into full power, if it is expected to be
+ * used on system resume. To distinguish that, we check whether the runtime PM
+ * usage count is greater than 1 (the PM core increases the usage count in the
+ * system PM prepare phase), as that indicates a real user (such as a subsystem,
+ * driver, userspace, etc.) is using it. If that is the case, the device is
+ * expected to be used on system resume as well, so then we resume it. In the
+ * other case, we defer the resume to be managed via runtime PM.
  *
- * Typically this function may be invoked from a system resume callback to make
- * sure the device is put into full power state.
+ * Typically this function may be invoked from a system resume callback.
  */
 int pm_runtime_force_resume(struct device *dev)
 {
@@ -1524,6 +1538,17 @@ int pm_runtime_force_resume(struct device *dev)
 	if (!pm_runtime_status_suspended(dev))
 		goto out;
 
+	/*
+	 * Decrease the parent's runtime PM usage count, if we increased it
+	 * during system suspend in pm_runtime_force_suspend().
+	*/
+	if (atomic_read(&dev->power.usage_count) > 1) {
+		if (dev->parent)
+			pm_runtime_put_noidle(dev->parent);
+	} else {
+		goto out;
+	}
+
 	ret = pm_runtime_set_active(dev);
 	if (ret)
 		goto out;