linux-kernel - [PATCH 3.16 052/305] PM / devfreq: Fix devfreq_add_device() when drivers are built as modules.

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Date:   Sun, 03 Feb 2019 14:45:08 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, Denis Kirjanov <kda@...ux-powerpc.org>,
        "MyungJoo Ham" <myungjoo.ham@...sung.com>,
        "Chanwoo Choi" <cw00.choi@...sung.com>,
        "Enric Balletbo i Serra" <enric.balletbo@...labora.com>
Subject: [PATCH 3.16 052/305] PM / devfreq: Fix devfreq_add_device() when
 drivers are built as modules.

3.16.63-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Enric Balletbo i Serra <enric.balletbo@...labora.com>

commit 23c7b54ca1cd1797ef39169ab85e6d46f1c2d061 upstream.

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@...labora.com>
Reviewed-by: Chanwoo Choi <cw00.choi@...sung.com>
Signed-off-by: MyungJoo Ham <myungjoo.ham@...sung.com>
[bwh: Backported to 3.16:
 - Use string literal instead of DEVFREQ_GOV_SIMPLE_ONDEMAND
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 drivers/devfreq/devfreq.c | 53 ++++++++++++++++++++++++++++++++++++---
 1 file changed, 49 insertions(+), 4 deletions(-)

--- 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>
@@ -148,6 +149,49 @@ static struct devfreq_governor *find_dev
 	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, "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;
+}
+
 /* Load monitoring helper functions for governors use */
 
 /**
@@ -499,9 +543,8 @@ struct devfreq *devfreq_add_device(struc
 	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__);
@@ -517,12 +560,14 @@ struct devfreq *devfreq_add_device(struc
 			__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);
@@ -798,7 +843,7 @@ static ssize_t governor_store(struct dev
 		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;