linux-kernel - Re: [PATCH 0/2] Add exit_prepare callback to the cpufreq

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Message-ID: <CAKohpokvpWDbCbPYcV+fzzpaztZSi=yacdmyYfBLoSs4U2YO9A@mail.gmail.com>
Date:	Thu, 4 Sep 2014 14:46:56 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
Cc:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Dirk Brandewie <dirk.brandewie@...il.com>,
	Dirk Brandewie <dirk.j.brandewie@...el.com>,
	Linux PM list <linux-pm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Patrick Marlier <patrick.marlier@...il.com>
Subject: Re: [PATCH 0/2] Add exit_prepare callback to the cpufreq_driver interface.

On 4 September 2014 14:40, Preeti U Murthy <preeti@...ux.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 <preeti@...ux.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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