linux-kernel - Re: [PATCH] cpufreq: Set policy to non-NULL only after all hotplug online work is done

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Message-ID: <CAKohpo==WVKytPCaMk=O5rGfPNFDAF7ffYo7OE0d3uAPyJfwwA@mail.gmail.com>
Date:	Wed, 26 Feb 2014 11:32:19 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Saravana Kannan <skannan@...eaurora.org>
Cc:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-arm-msm@...r.kernel.org,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] cpufreq: Set policy to non-NULL only after all hotplug
 online work is done

On 26 February 2014 07:18, Saravana Kannan <skannan@...eaurora.org> wrote:
> On 02/25/2014 02:41 PM, Rafael J. Wysocki wrote:

>> And is "fully initialized" actually well defined?
>
> The point in add dev/hot plug path after which we will no longer change
> policy fields without sending further CPUFREQ_UPDATE_POLICY_CPU /
> CPUFRE_NOTIFY notifiers.

Okay..

> Pretty much the end of __cpufreq_add_dev() so that it's after:
> - cpufreq_init_policy()
> - And the update of userpolicy fields that after thie init call

No. In that case it can be considered initialized before cpufreq_init_policy().
As we do send CPUFREQ_NOTIFY after that from cpufreq_init_policy()->
cpufreq_set_policy().

There are two types of fields within policy, some are very basic: cpu/min/max/
affected_cpus/related_cpus

some are advanced: sysfs/governors/..

And as a rule you have to get policy->rwsem lock before accessing policy
members. We might not have followed it very well for small things like cpu.

And so if you are doing anything over that, please use a lock and that is
already present in cpufreq_update_policy().

With my latest patchset that I sent yesterday, locking is improved and now
a policy will be usable only after the rwsem is released. And that should be
fine. And so making it available in the per-cpu variable after all the necessary
fields are filled looks fine to me. And so I don't think we need to move it
after call to cpufreq_init_policy(maybe a better name to this function is
required)..

> Ok, here's some pseudo code to explain it better:
>
> Something like, replace all calls to cpufreq_driver->get with
> __cpufreq_driver_get() with the fn being something like:
>
> unsigned int __cpufreq_driver_get(cpufreq_policy *policy)
> {
>   if (policy->clk)
>      return clk_get_rate(policy->clk) / 1000;
>   else
>      return cpufreq_driver->get(policy->cpu);

This part may still use cpufreq_cpu_get().

> }

Drivers are free to have their implementation of ->get() even
if they have a valid policy->clk field..
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