devfreq: devfreq_simple_dev: Add support for preparing device clock

For certain implementation, device clock needs to be prepared before
rate voting taking effect. Add support for preparing device clock
during initialization.

Change-Id: Ib22e83952187118342ff2546d4c79d3970a288f9
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>

Documentation/devicetree/bindings/devfreq/devfreq-simple-dev.txt

@@ -14,6 +14,7 @@ Required properties:
 Optional properties:
 - polling-ms:	Polling interval for the device in milliseconds. Default: 50
 - governor:	Initial governor to user for the device. Default: "performance"
+- qcom,prepare-clk:	Prepare the device clock during initialization.
 
 Example:
 

drivers/devfreq/devfreq_simple_dev.c

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2014, The Linux Foundation. All rights reserved.
+ * Copyright (c) 2014-2015, The Linux Foundation. All rights reserved.
  *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 2 and
@@ -146,12 +146,23 @@ static int devfreq_clock_probe(struct platform_device *pdev)
 	if (of_property_read_string(dev->of_node, "governor", &gov_name))
 		gov_name = "performance";
 
+	if (of_property_read_bool(dev->of_node, "qcom,prepare-clk")) {
+		ret = clk_prepare(d->clk);
+		if (ret)
+			return ret;
+	}
 
 	d->df = devfreq_add_device(dev, p, gov_name, NULL);
-	if (IS_ERR(d->df))
-		return PTR_ERR(d->df);
+	if (IS_ERR(d->df)) {
+		ret = PTR_ERR(d->df);
+		goto add_err;
+	}
 
 	return 0;
+add_err:
+	if (of_property_read_bool(dev->of_node, "qcom,prepare-clk"))
+		clk_unprepare(d->clk);
+	return ret;
 }
 
 static int devfreq_clock_remove(struct platform_device *pdev)