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

When the devfreq driver and the governor driver are built as modules,
the call to devfreq_add_device() or governor_store() 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 by adding a try_then_request_governor()
function. First tries to find the governor, and then, if it is 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>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>

1 files changed, 49 insertions, 4 deletions

diff --git a/drivers/devfreq/devfreq.c b/drivers/devfreq/devfreq.c
index 4c49bb1330b5..62ead442a872 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>
@@ -221,6 +222,49 @@ static struct devfreq_governor *find_devfreq_governor(const char *name)
         return ERR_PTR(-ENODEV);
 }
 
+/**
+ * try_then_request_governor() - Try to find the governor and request the
+ *                               module if is not found.
+ * @name:       name of the governor
+ *
+ * Search the list of devfreq governors and request the module and try again
+ * if is not found. This can happen when both drivers (the governor driver
+ * and the driver that call devfreq_add_device) are built as modules.
+ * devfreq_list_lock should be held by the caller. Returns the matched
+ * governor's pointer.
+ */
+static struct devfreq_governor *try_then_request_governor(const char *name)
+{
+        struct devfreq_governor *governor;
+        int err = 0;
+
+        if (IS_ERR_OR_NULL(name)) {
+                pr_err("DEVFREQ: %s: Invalid parameters\n", __func__);
+                return ERR_PTR(-EINVAL);
+        }
+        WARN(!mutex_is_locked(&devfreq_list_lock),
+             "devfreq_list_lock must be locked.");
+
+        governor = find_devfreq_governor(name);
+        if (IS_ERR(governor)) {
+                mutex_unlock(&devfreq_list_lock);
+
+                if (!strncmp(name, DEVFREQ_GOV_SIMPLE_ONDEMAND,
+                             DEVFREQ_NAME_LEN))
+                        err = request_module("governor_%s", "simpleondemand");
+                else
+                        err = request_module("governor_%s", name);
+                /* Restore previous state before return */
+                mutex_lock(&devfreq_list_lock);
+                if (err)
+                        return NULL;
+
+                governor = find_devfreq_governor(name);
+        }
+
+        return governor;
+}
+
 static int devfreq_notify_transition(struct devfreq *devfreq,
                 struct devfreq_freqs *freqs, unsigned int state)
 {
@@ -646,9 +690,8 @@ struct devfreq *devfreq_add_device(struct device *dev,
         mutex_unlock(&devfreq->lock);
 
         mutex_lock(&devfreq_list_lock);
-        list_add(&devfreq->node, &devfreq_list);
 
-        governor = find_devfreq_governor(devfreq->governor_name);
+        governor = try_then_request_governor(devfreq->governor_name);
         if (IS_ERR(governor)) {
                 dev_err(dev, "%s: Unable to find governor for the device\n",
                         __func__);
@@ -664,12 +707,14 @@ struct devfreq *devfreq_add_device(struct device *dev,
                         __func__);
                 goto err_init;
         }
+
+        list_add(&devfreq->node, &devfreq_list);
+
         mutex_unlock(&devfreq_list_lock);
 
         return devfreq;
 
 err_init:
-        list_del(&devfreq->node);
         mutex_unlock(&devfreq_list_lock);
 
         device_unregister(&devfreq->dev);
@@ -991,7 +1036,7 @@ static ssize_t governor_store(struct device *dev, struct device_attribute *attr,
                 return -EINVAL;
 
         mutex_lock(&devfreq_list_lock);
-        governor = find_devfreq_governor(str_governor);
+        governor = try_then_request_governor(str_governor);
         if (IS_ERR(governor)) {
                 ret = PTR_ERR(governor);
                 goto out;