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

[Sudeep Holla]

On Wed, Nov 27, 2019 at 12:48:01PM +0100, Dietmar Eggemann wrote:
> 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.
>

Unlike DT based drivers, it not easy to get the fixed cpumask unless we
add some mechanism to extract it based on clks/OPP added. I prefer
this simple solution instead.

> 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
>

Reviewed-by: Sudeep Holla <sudeep.holla@xxxxxxx>
Tested-by: Sudeep Holla <sudeep.holla@xxxxxxx>

--
Regards,
Sudeep