On 4 September 2014 14:40, Preeti U Murthy <pre...@linux.vnet.ibm.com> wrote:
> cpufreq: Allow stop CPU callback to be used by all cpufreq drivers
>
> Commit 367dc4aa introduced the stop CPU callback for intel_pstate
> drivers. During the CPU_DOWN_PREPARE stage, this callback is invoked
> so that drivers can take some action on the pstate of the cpu
> before it is taken offline. This callback was assumed to be useful
> only for those drivers which have implemented the set_policy CPU
> callback because they have no other way to take action about the
> cpufreq of a CPU which is being hotplugged out except in the exit
> callback which is called very late in the offline process.
>
> The drivers which implement the target/target_index callbacks were
> expected to take care of requirements like the ones that commit
> 367dc4aa addresses in the GOV_STOP notification event. But there
> are disadvantages to restricting the usage of stop CPU callback
> to cpufreq drivers that implement the set_policy callbacks and who
> want to take explicit action on the setting the cpufreq during a
> hotplug operation.
>
> 1.GOV_STOP gets called for every CPU offline and drivers would usually
> want to take action when the last cpu in the policy->cpus mask
> is taken offline. As long as there is more than one cpu in the
> policy->cpus mask, cpufreq core itself makes sure that the freq
> for the other cpus in this mask is set according to the maximum load.
> This is sensible and drivers which implement the target_index callback
> would mostly not want to modify that. However the cpufreq core leaves a
> loose end when the cpu in the policy->cpus mask is the last one to go
> offline;
> it does nothing explicit to the frequency of the core. Drivers may need
> a way to take some action here and stop CPU callback mechanism is the
> best way to do it today.
>
> 2.We cannot implement driver specific actions in the GOV_STOP mechanism.
> So we will need another driver callback which is invoked from here which is
> unnecessary.
>
> Therefore this patch extends the usage of stop CPU callback to be used
> by all cpufreq drivers as long as they have this callback implemented
> and irrespective of whether they are set_policy/target_index drivers.
> The assumption is if the drivers find the GOV_STOP path to be a suitable
> way of implementing what they want to do with the freq of the cpu
> going offine,they will not implement the stop CPU callback at all.
>
> Signed-off-by: Preeti U Murthy <pre...@linux.vnet.ibm.com>
>
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index d9fdedd..6463f35 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -1380,7 +1380,7 @@ static int __cpufreq_remove_dev_prepare(struct device
> *dev,
> if (!cpufreq_suspended)
> pr_debug("%s: policy Kobject moved to cpu: %d from:
> %d\n",
> __func__, new_cpu, cpu);
> - } else if (cpufreq_driver->stop_cpu && cpufreq_driver->setpolicy) {
> + } else if (cpufreq_driver->stop_cpu) {
> cpufreq_driver->stop_cpu(policy);
> }
Rafael explicitly said earlier that he want to see a separate callback for
->target() drivers, don't know why..
It looks fine to me though.
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