[PATCH] PM / devfreq: Fix devfreq_add_device() when drivers are built as modules.

[Enric Balletbo i Serra <enric.balletbo@collabora.com>]

When the devfreq driver and the governor driver are built as modules,
the call to devfreq_add_device() fails because the governor driver is
not loaded at the time the devfreq driver loads. The devfreq driver has
a build dependency on the governor but also should have a runtime
dependency. We need to make sure that the governor driver is loaded
before the devfreq driver.

This patch fixes this bug in devfreq_add_device(). First tries to find
the governor, and then, if was not found, it requests the module and
tries again.

Fixes: 1b5c1be2c88e (PM / devfreq: map devfreq drivers to governor using name)
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
---

 drivers/devfreq/devfreq.c | 36 +++++++++++++++++++++++++++++++-----
 1 file changed, 31 insertions(+), 5 deletions(-)

diff --git a/drivers/devfreq/devfreq.c b/drivers/devfreq/devfreq.c
index fe2af6aa88fc..1d917f043e30 100644
--- a/drivers/devfreq/devfreq.c
+++ b/drivers/devfreq/devfreq.c
@@ -11,6 +11,7 @@
  */
 
 #include <linux/kernel.h>
+#include <linux/kmod.h>
 #include <linux/sched.h>
 #include <linux/errno.h>
 #include <linux/err.h>
@@ -648,10 +649,35 @@ struct devfreq *devfreq_add_device(struct device *dev,
 
 	governor = find_devfreq_governor(devfreq->governor_name);
 	if (IS_ERR(governor)) {
-		dev_err(dev, "%s: Unable to find governor for the device\n",
-			__func__);
-		err = PTR_ERR(governor);
-		goto err_init;
+		list_del(&devfreq->node);
+		mutex_unlock(&devfreq_list_lock);
+
+		/*
+		 * If the governor is not found, then request the module and
+		 * try again. This can happen when both drivers (the governor
+		 * driver and the driver that calls devfreq_add_device) are
+		 * built as modules.
+		 */
+		if (!strncmp(devfreq->governor_name,
+			     DEVFREQ_GOV_SIMPLE_ONDEMAND, DEVFREQ_NAME_LEN))
+			err = request_module("governor_%s", "simpleondemand");
+		else
+			err = request_module("governor_%s",
+					     devfreq->governor_name);
+		if (err)
+			goto err_unregister;
+
+		mutex_lock(&devfreq_list_lock);
+		list_add(&devfreq->node, &devfreq_list);
+
+		governor = find_devfreq_governor(devfreq->governor_name);
+		if (IS_ERR(governor)) {
+			dev_err(dev,
+				"%s: Unable to find governor for the device\n",
+				__func__);
+			err = PTR_ERR(governor);
+			goto err_init;
+		}
 	}
 
 	devfreq->governor = governor;
@@ -669,7 +695,7 @@ struct devfreq *devfreq_add_device(struct device *dev,
 err_init:
 	list_del(&devfreq->node);
 	mutex_unlock(&devfreq_list_lock);
-
+err_unregister:
 	device_unregister(&devfreq->dev);
 err_dev:
 	if (devfreq)
-- 
2.17.1