[PATCH] cpufreq: vexpress-spc: Fix wrong alternation of policy->related_cpus during CPU hp

[Dietmar Eggemann]

Since commit ca74b316df96 ("arm: Use common cpu_topology structure and
functions.") the core cpumask has to be modified during cpu hotplug
operations.

("arm: Fix topology setup in case of CPU hotplug for CONFIG_SCHED_MC")
[1] fixed that but revealed another issue on TC2, i.e in its cpufreq
driver.

During CPU hp stress operations on multiple CPUs, policy->related_cpus
can be altered. This is wrong since this cpumask should contain the
online and offline CPUs.

The WARN_ON(!cpumask_test_cpu(cpu, policy->related_cpus)) in
cpufreq_online() triggers in this case.

The core cpumask can't be used to set the policy->cpus in
ve_spc_cpufreq_init() anymore in case it is called via
cpuhp_cpufreq_online()->cpufreq_online()->cpufreq_driver->init().

An empty online() callback can be used to avoid that the init()
driver function is called during CPU hotplug in so that
policy->related_cpus will not be changed.

Implementing an online() also requires an offline() callback.

Tested on TC2 with CPU hp stress test (CPU hp from multiple CPUs at
the same time).

[1]
https://lore.kernel.org/r/20191127103353.12417-1-dietmar.eggemann@xxxxxxx

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
---
 drivers/cpufreq/vexpress-spc-cpufreq.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/drivers/cpufreq/vexpress-spc-cpufreq.c b/drivers/cpufreq/vexpress-spc-cpufreq.c
index 506e3f2bf53a..857dcb535e97 100644
--- a/drivers/cpufreq/vexpress-spc-cpufreq.c
+++ b/drivers/cpufreq/vexpress-spc-cpufreq.c
@@ -492,6 +492,16 @@ static void ve_spc_cpufreq_ready(struct cpufreq_policy *policy)
 	cdev[cur_cluster] = of_cpufreq_cooling_register(policy);
 }
 
+static int ve_spc_cpufreq_online(struct cpufreq_policy *policy)
+{
+	return 0;
+}
+
+static int ve_spc_cpufreq_offline(struct cpufreq_policy *policy)
+{
+	return 0;
+}
+
 static struct cpufreq_driver ve_spc_cpufreq_driver = {
 	.name			= "vexpress-spc",
 	.flags			= CPUFREQ_STICKY |
@@ -503,6 +513,8 @@ static struct cpufreq_driver ve_spc_cpufreq_driver = {
 	.init			= ve_spc_cpufreq_init,
 	.exit			= ve_spc_cpufreq_exit,
 	.ready			= ve_spc_cpufreq_ready,
+	.online                 = ve_spc_cpufreq_online,
+	.offline                = ve_spc_cpufreq_offline,
 	.attr			= cpufreq_generic_attr,
 };
 
-- 
2.17.1