[PATCH 1/6] cpufreq: cpufreq-cpu0: allow optional safe voltage during frequency transitions

[Thomas Abraham <ta.omasab@gmail.com>]

On some platforms such as the Samsung Exynos, changing the frequency
of the CPU clock requires changing the frequency of the PLL that is
supplying the CPU clock. To change the frequency of the PLL, the CPU
clock is temporarily reparented to another parent clock.

The clock frequency of this temporary parent clock could be much higher
than the clock frequency of the PLL at the time of reparenting. Due
to the temporary increase in the CPU clock speed, the CPU (and any other
components in the CPU clock domain such as dividers, mux, etc.) have to
to be operated at a higher voltage level, called the safe voltage level.
This patch adds optional support to temporarily switch to a safe voltage
level during CPU frequency transitions.

Cc: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Thomas Abraham <thomas.ab@samsung.com>
---
 .../devicetree/bindings/cpufreq/cpufreq-cpu0.txt   |    5 ++
 drivers/cpufreq/cpufreq-cpu0.c                     |   49 +++++++++++++++++++-
 2 files changed, 52 insertions(+), 2 deletions(-)

diff --git a/Documentation/devicetree/bindings/cpufreq/cpufreq-cpu0.txt b/Documentation/devicetree/bindings/cpufreq/cpufreq-cpu0.txt
index f055515..020d859 100644
--- a/Documentation/devicetree/bindings/cpufreq/cpufreq-cpu0.txt
+++ b/Documentation/devicetree/bindings/cpufreq/cpufreq-cpu0.txt
@@ -19,6 +19,10 @@ Optional properties:
 - cooling-min-level:
 - cooling-max-level:
      Please refer to Documentation/devicetree/bindings/thermal/thermal.txt.
+- safe-opp-index: Certain platforms require that during a opp transition,
+  a system should not go below a particular opp level. For such systems,
+  this property specifies the minimum opp to be maintained during the
+  opp transitions.
 
 Examples:
 
@@ -36,6 +40,7 @@ cpus {
 			396000  950000
 			198000  850000
 		>;
+		safe-opp-index = <1>;
 		clock-latency = <61036>; /* two CLK32 periods */
 		#cooling-cells = <2>;
 		cooling-min-level = <0>;
diff --git a/drivers/cpufreq/cpufreq-cpu0.c b/drivers/cpufreq/cpufreq-cpu0.c
index 0c12ffc..dda4b7b 100644
--- a/drivers/cpufreq/cpufreq-cpu0.c
+++ b/drivers/cpufreq/cpufreq-cpu0.c
@@ -27,6 +27,8 @@
 
 static unsigned int transition_latency;
 static unsigned int voltage_tolerance; /* in percentage */
+static unsigned long safe_frequency;
+static unsigned long safe_voltage;
 
 static struct device *cpu_dev;
 static struct clk *cpu_clk;
@@ -69,12 +71,26 @@ static int cpu0_set_target(struct cpufreq_policy *policy, unsigned int index)
 		 new_freq / 1000, volt ? volt / 1000 : -1);
 
 	/* scaling up?  scale voltage before frequency */
-	if (!IS_ERR(cpu_reg) && new_freq > old_freq) {
+	if (!IS_ERR(cpu_reg) && new_freq > old_freq &&
+				new_freq >= safe_frequency) {
 		ret = regulator_set_voltage_tol(cpu_reg, volt, tol);
 		if (ret) {
 			pr_err("failed to scale voltage up: %d\n", ret);
 			return ret;
 		}
+	} else if (!IS_ERR(cpu_reg) && old_freq < safe_frequency) {
+		/*
+		 * the scaled up voltage level for the new_freq is lower
+		 * than the safe voltage level. so set safe_voltage
+		 * as the intermediate voltage level and revert it
+		 * back after the frequency has been changed.
+		 */
+		ret = regulator_set_voltage(cpu_reg, safe_voltage,
+						safe_voltage);
+		if (ret) {
+			pr_err("failed to set safe voltage: %d\n", ret);
+			return ret;
+		}
 	}
 
 	ret = clk_set_rate(cpu_clk, freq_exact);
@@ -94,6 +110,19 @@ static int cpu0_set_target(struct cpufreq_policy *policy, unsigned int index)
 		}
 	}
 
+	/*
+	 * if safe voltage was applied during voltage scale up, then set
+	 * the correct target voltage now.
+	 */
+	if (!IS_ERR(cpu_reg) && new_freq > old_freq &&
+					new_freq < safe_frequency) {
+		ret = regulator_set_voltage_tol(cpu_reg, volt, tol);
+		if (ret) {
+			pr_err("failed to scale voltage up: %d\n", ret);
+			return ret;
+		}
+	}
+
 	return ret;
 }
 
@@ -116,7 +145,9 @@ static struct cpufreq_driver cpu0_cpufreq_driver = {
 
 static int cpu0_cpufreq_probe(struct platform_device *pdev)
 {
+	struct dev_pm_opp *opp;
 	struct device_node *np;
+	unsigned int safe_opp_index;
 	int ret;
 
 	cpu_dev = get_cpu_device(0);
@@ -165,13 +196,27 @@ static int cpu0_cpufreq_probe(struct platform_device *pdev)
 		goto out_put_node;
 	}
 
+	if (!of_property_read_u32(np, "safe-opp-index", &safe_opp_index)) {
+		rcu_read_lock();
+		opp = dev_pm_opp_find_freq_exact(cpu_dev,
+			freq_table[safe_opp_index].frequency * 1000, true);
+		if (IS_ERR(opp)) {
+			rcu_read_unlock();
+			pr_err("safe opp index %d is invalid\n",
+							safe_opp_index);
+			goto out_free_table;
+		}
+		safe_voltage = dev_pm_opp_get_voltage(opp);
+		safe_frequency = freq_table[safe_opp_index].frequency;
+		rcu_read_unlock();
+	}
+
 	of_property_read_u32(np, "voltage-tolerance", &voltage_tolerance);
 
 	if (of_property_read_u32(np, "clock-latency", &transition_latency))
 		transition_latency = CPUFREQ_ETERNAL;
 
 	if (!IS_ERR(cpu_reg)) {
-		struct dev_pm_opp *opp;
 		unsigned long min_uV, max_uV;
 		int i;
 
-- 
1.6.6.rc2