[PATCH] thermal: cpu_cooling: always use first CPU of a freq domain as cooling device

[Matthias Kaehlcke]

For all CPUs of a frequency domain a single cooling device is
registered, since the CPUs can't switch their frequencies
independently from each other. The cpufreq policy CPU is used to
represent cooling device of the frequency domain. Which CPU is the
policy CPU may vary based on the order of initialization or CPU
hotplug.

For device tree based platform the above implies that cooling maps
must include a list of all possible cooling devices of a frequency
domain, even though only one of them will exist at any given time.

For example:

cooling-maps {
	map0 {
		trip = <&cpu_alert0>;
		cooling-device = <&CPU0 THERMAL_NO_LIMIT 4>,
				 <&CPU1 THERMAL_NO_LIMIT 4>,
				 <&CPU2 THERMAL_NO_LIMIT 4>,
				 <&CPU3 THERMAL_NO_LIMIT 4>;
	};
	map1 {
		trip = <&cpu_crit0>;
		cooling-device = <&CPU0 THERMAL_NO_LIMIT THERMAL_NO_LIMIT>,
				 <&CPU1 THERMAL_NO_LIMIT THERMAL_NO_LIMIT>,
				 <&CPU2 THERMAL_NO_LIMIT THERMAL_NO_LIMIT>,
				 <&CPU3 THERMAL_NO_LIMIT THERMAL_NO_LIMIT>;
	};
};

This can be avoided by using always the first CPU of a frequency
domain as cooling device. It may happen that the first CPU is offline
when the cooling device is registered (e.g. CPU2 is initialized
first in the above example), hence the nominal cooling device might
be offline. This may seem odd, however it is not really different from
the current behavior: when the policy CPU is taking offline the cooling
device corresponding to it remains active, unless it is unregistered
because all other CPUs of the frequency domain are offline too.

A single cooling device associated with a specific CPU of the frequency
domain reduces redundant device tree clutter in CPU nodes and cooling
maps.

Signed-off-by: Matthias Kaehlcke <mka@xxxxxxxxxxxx>
---
 drivers/thermal/cpu_cooling.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/drivers/thermal/cpu_cooling.c b/drivers/thermal/cpu_cooling.c
index dfd23245f778a..bb5ea06f893a2 100644
--- a/drivers/thermal/cpu_cooling.c
+++ b/drivers/thermal/cpu_cooling.c
@@ -758,13 +758,14 @@ EXPORT_SYMBOL_GPL(cpufreq_cooling_register);
 struct thermal_cooling_device *
 of_cpufreq_cooling_register(struct cpufreq_policy *policy)
 {
-	struct device_node *np = of_get_cpu_node(policy->cpu, NULL);
+	unsigned int first_cpu = cpumask_first(policy->related_cpus);
+	struct device_node *np = of_get_cpu_node(first_cpu, NULL);
 	struct thermal_cooling_device *cdev = NULL;
 	u32 capacitance = 0;
 
 	if (!np) {
 		pr_err("cpu_cooling: OF node not available for cpu%d\n",
-		       policy->cpu);
+		       first_cpu);
 		return NULL;
 	}
 
@@ -775,7 +776,7 @@ of_cpufreq_cooling_register(struct cpufreq_policy *policy)
 		cdev = __cpufreq_cooling_register(np, policy, capacitance);
 		if (IS_ERR(cdev)) {
 			pr_err("cpu_cooling: cpu%d is not running as cooling device: %ld\n",
-			       policy->cpu, PTR_ERR(cdev));
+			       first_cpu, PTR_ERR(cdev));
 			cdev = NULL;
 		}
 	}


Would that make sense or is there something I'm overlooking?

Cheers

Matthias