[CPUFREQ] ARM Exynos4210 PM/Suspend compatibility with different bootloaders

We have various bootloaders for Exynos4210 machines. Some of they
set the ARM core frequency at boot time even when the boot is a resume
from suspend-to-RAM. Such changes may create inconsistency in the
data of CPUFREQ driver and have incurred hang issues with suspend-to-RAM.

This patch enables to save and restore CPU frequencies with pm-notifier and
sets the frequency at the initial (boot-time) value so that there wouldn't
be any inconsistency between bootloader and kernel. This patch does not
use CPUFREQ's suspend/resume callbacks because they are syscore-ops, which
do not allow to use mutex that is being used by regulators that are used by
the target function.

This also prevents any CPUFREQ transitions during suspend-resume context,
which could be dangerous at noirq-context along with regulator framework.

Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Dave Jones <davej@redhat.com>

drivers/cpufreq/exynos4210-cpufreq.c

@@ -17,6 +17,8 @@
 #include <linux/slab.h>
 #include <linux/regulator/consumer.h>
 #include <linux/cpufreq.h>
+#include <linux/notifier.h>
+#include <linux/suspend.h>
 
 #include <mach/map.h>
 #include <mach/regs-clock.h>
@@ -36,6 +38,10 @@ static struct regulator *int_regulator;
 static struct cpufreq_freqs freqs;
 static unsigned int memtype;
 
+static unsigned int locking_frequency;
+static bool frequency_locked;
+static DEFINE_MUTEX(cpufreq_lock);
+
 enum exynos4_memory_type {
 	DDR2 = 4,
 	LPDDR2,
@@ -405,22 +411,32 @@ static int exynos4_target(struct cpufreq_policy *policy,
 {
 	unsigned int index, old_index;
 	unsigned int arm_volt, int_volt;
+	int err = -EINVAL;
 
 	freqs.old = exynos4_getspeed(policy->cpu);
 
+	mutex_lock(&cpufreq_lock);
+
+	if (frequency_locked && target_freq != locking_frequency) {
+		err = -EAGAIN;
+		goto out;
+	}
+
 	if (cpufreq_frequency_table_target(policy, exynos4_freq_table,
 					   freqs.old, relation, &old_index))
-		return -EINVAL;
+		goto out;
 
 	if (cpufreq_frequency_table_target(policy, exynos4_freq_table,
 					   target_freq, relation, &index))
-		return -EINVAL;
+		goto out;
+
+	err = 0;
 
 	freqs.new = exynos4_freq_table[index].frequency;
 	freqs.cpu = policy->cpu;
 
 	if (freqs.new == freqs.old)
-		return 0;
+		goto out;
 
 	/* get the voltage value */
 	arm_volt = exynos4_volt_table[index].arm_volt;
@@ -447,10 +463,16 @@ static int exynos4_target(struct cpufreq_policy *policy,
 
 	cpufreq_notify_transition(&freqs, CPUFREQ_POSTCHANGE);
 
-	return 0;
+out:
+	mutex_unlock(&cpufreq_lock);
+	return err;
 }
 
 #ifdef CONFIG_PM
+/*
+ * These suspend/resume are used as syscore_ops, it is already too
+ * late to set regulator voltages at this stage.
+ */
 static int exynos4_cpufreq_suspend(struct cpufreq_policy *policy)
 {
 	return 0;
@@ -462,6 +484,78 @@ static int exynos4_cpufreq_resume(struct cpufreq_policy *policy)
 }
 #endif
 
+/**
+ * exynos4_cpufreq_pm_notifier - block CPUFREQ's activities in suspend-resume
+ *			context
+ * @notifier
+ * @pm_event
+ * @v
+ *
+ * While frequency_locked == true, target() ignores every frequency but
+ * locking_frequency. The locking_frequency value is the initial frequency,
+ * which is set by the bootloader. In order to eliminate possible
+ * inconsistency in clock values, we save and restore frequencies during
+ * suspend and resume and block CPUFREQ activities. Note that the standard
+ * suspend/resume cannot be used as they are too deep (syscore_ops) for
+ * regulator actions.
+ */
+static int exynos4_cpufreq_pm_notifier(struct notifier_block *notifier,
+				       unsigned long pm_event, void *v)
+{
+	struct cpufreq_policy *policy = cpufreq_cpu_get(0); /* boot CPU */
+	static unsigned int saved_frequency;
+	unsigned int temp;
+
+	mutex_lock(&cpufreq_lock);
+	switch (pm_event) {
+	case PM_SUSPEND_PREPARE:
+		if (frequency_locked)
+			goto out;
+		frequency_locked = true;
+
+		if (locking_frequency) {
+			saved_frequency = exynos4_getspeed(0);
+
+			mutex_unlock(&cpufreq_lock);
+			exynos4_target(policy, locking_frequency,
+				       CPUFREQ_RELATION_H);
+			mutex_lock(&cpufreq_lock);
+		}
+
+		break;
+	case PM_POST_SUSPEND:
+
+		if (saved_frequency) {
+			/*
+			 * While frequency_locked, only locking_frequency
+			 * is valid for target(). In order to use
+			 * saved_frequency while keeping frequency_locked,
+			 * we temporarly overwrite locking_frequency.
+			 */
+			temp = locking_frequency;
+			locking_frequency = saved_frequency;
+
+			mutex_unlock(&cpufreq_lock);
+			exynos4_target(policy, locking_frequency,
+				       CPUFREQ_RELATION_H);
+			mutex_lock(&cpufreq_lock);
+
+			locking_frequency = temp;
+		}
+
+		frequency_locked = false;
+		break;
+	}
+out:
+	mutex_unlock(&cpufreq_lock);
+
+	return NOTIFY_OK;
+}
+
+static struct notifier_block exynos4_cpufreq_nb = {
+	.notifier_call = exynos4_cpufreq_pm_notifier,
+};
+
 static int exynos4_cpufreq_cpu_init(struct cpufreq_policy *policy)
 {
 	int ret;
@@ -522,6 +616,8 @@ static int __init exynos4_cpufreq_init(void)
 	if (IS_ERR(cpu_clk))
 		return PTR_ERR(cpu_clk);
 
+	locking_frequency = exynos4_getspeed(0);
+
 	moutcore = clk_get(NULL, "moutcore");
 	if (IS_ERR(moutcore))
 		goto out;
@@ -561,6 +657,8 @@ static int __init exynos4_cpufreq_init(void)
 		printk(KERN_DEBUG "%s: memtype= 0x%x\n", __func__, memtype);
 	}
 
+	register_pm_notifier(&exynos4_cpufreq_nb);
+
 	return cpufreq_register_driver(&exynos4_driver);
 
 out: