cpufreq: OMAP: instantiate omap-cpufreq as a platform_driver

As multi-platform build is being adopted by more and more ARM platforms,
initcall function should be used very carefully.  For example, when
CONFIG_ARM_OMAP2PLUS_CPUFREQ is built in the kernel, omap_cpufreq_init()
will be called on all the platforms to initialize omap-cpufreq driver.

Further, on OMAP, we now use Soc generic cpufreq-cpu0 driver using device
tree entries.  To allow cpufreq-cpu0 and omap-cpufreq drivers to co-exist
for OMAP in a single image, we need to ensure the following:
 1. With device tree boot, we use cpufreq-cpu0
 2. With non device tree boot, we use omap-cpufreq

In the case of (1), we will have cpu OPPs and regulator registered
as part of the device tree nodes, to ensure that omap-cpufreq
and cpufreq-cpu0 don't conflict in managing the frequency of the
same CPU, we should not permit omap-cpufreq to be probed.

In the case of (2), we will not have the cpufreq-cpu0 device, hence
only omap-cpufreq will be active.

To eliminate this undesired these effects, we change omap-cpufreq
driver to have it instantiated as a platform_driver and register
"omap-cpufreq" device only when booted without device tree nodes on
OMAP platforms.

This allows the following:
 a) Will only run on platforms that create the platform_device
    "omap-cpufreq".
 b) Since the platform_device is registered only when device tree nodes
    are *not* populated, omap-cpufreq driver does not conflict with
    the usage of cpufreq-cpu0 driver which is used on OMAP platforms when
    device tree nodes are present.

Inspired by commit 5553f9e2
(cpufreq: instantiate cpufreq-cpu0 as a platform_driver)

[robherring2@gmail.com: reported conflict of omap-cpufreq vs other
driver in an non-device tree supported boot]
Reported-by: Rob Herring <robherring2@gmail.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

arch/arm/mach-omap2/pm.c

@@ -265,6 +265,12 @@ static void __init omap4_init_voltages(void)
 	omap2_set_init_voltage("iva", "dpll_iva_m5x2_ck", "iva");
 }
 
+static inline void omap_init_cpufreq(void)
+{
+	struct platform_device_info devinfo = { .name = "omap-cpufreq", };
+	platform_device_register_full(&devinfo);
+}
+
 static int __init omap2_common_pm_init(void)
 {
 	if (!of_have_populated_dt())
@@ -294,6 +300,9 @@ int __init omap2_common_pm_late_init(void)
 
 		/* Smartreflex device init */
 		omap_devinit_smartreflex();
+
+		/* cpufreq dummy device instantiation */
+		omap_init_cpufreq();
 	}
 
 #ifdef CONFIG_SUSPEND

drivers/cpufreq/omap-cpufreq.c

@@ -25,6 +25,7 @@
 #include <linux/opp.h>
 #include <linux/cpu.h>
 #include <linux/module.h>
+#include <linux/platform_device.h>
 #include <linux/regulator/consumer.h>
 
 #include <asm/smp_plat.h>
@@ -243,7 +244,7 @@ static struct cpufreq_driver omap_driver = {
 	.attr		= omap_cpufreq_attr,
 };
 
-static int __init omap_cpufreq_init(void)
+static int omap_cpufreq_probe(struct platform_device *pdev)
 {
 	mpu_dev = get_cpu_device(0);
 	if (!mpu_dev) {
@@ -271,12 +272,20 @@ static int __init omap_cpufreq_init(void)
 	return cpufreq_register_driver(&omap_driver);
 }
 
-static void __exit omap_cpufreq_exit(void)
+static int omap_cpufreq_remove(struct platform_device *pdev)
 {
-	cpufreq_unregister_driver(&omap_driver);
+	return cpufreq_unregister_driver(&omap_driver);
 }
 
+static struct platform_driver omap_cpufreq_platdrv = {
+	.driver = {
+		.name	= "omap-cpufreq",
+		.owner	= THIS_MODULE,
+	},
+	.probe		= omap_cpufreq_probe,
+	.remove		= omap_cpufreq_remove,
+};
+module_platform_driver(omap_cpufreq_platdrv);
+
 MODULE_DESCRIPTION("cpufreq driver for OMAP SoCs");
 MODULE_LICENSE("GPL");
-module_init(omap_cpufreq_init);
-module_exit(omap_cpufreq_exit);