linux-kernel - Re: [PATCH v3 1/2] cpufreq: Don't destroy/realloc policy/sysfs on hotplug/suspend

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Date:	Wed, 16 Jul 2014 18:43:45 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	"Srivatsa S. Bhat" <srivatsa@....edu>
Cc:	Saravana Kannan <skannan@...eaurora.org>,
	"Rafael J . Wysocki" <rjw@...ysocki.net>,
	Todd Poynor <toddpoynor@...gle.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-msm@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Stephen Boyd <sboyd@...eaurora.org>
Subject: Re: [PATCH v3 1/2] cpufreq: Don't destroy/realloc policy/sysfs on hotplug/suspend

On 16 July 2014 16:46, Srivatsa S. Bhat <srivatsa@....edu> wrote:
> Short answer: If the sysfs directory has already been created by cpufreq,
> then yes, it will remain as it is. However, if the online operation failed
> before that, then cpufreq won't know about that CPU at all, and no file will
> be created.
>
> Long answer:
> The existing cpufreq code does all its work (including creating the sysfs
> directories etc) at the CPU_ONLINE stage. This stage is not expected to fail
> (in fact even the core CPU hotplug code in kernel/cpu.c doesn't care for
> error returns at this point). So if a CPU fails to come up in earlier stages
> itself (such as CPU_UP_PREPARE), then cpufreq won't even hear about that CPU,
> and hence no sysfs files will be created/linked. However, if the CPU bringup
> operation fails during the CPU_ONLINE stage after the cpufreq's notifier has
> been invoked, then we do nothing about it and the cpufreq sysfs files will
> remain.

In short, the problem I mentioned before this para is genuine. And setting
policy->cpu to the first cpu of a mask is indeed a bad idea.

>> Also, how does suspend/resume work without CONFIG_HOTPLUG_CPU ?
>> What's the sequence of events?
>>
>
> Well, CONFIG_SUSPEND doesn't have an explicit dependency on HOTPLUG_CPU, but
> SMP systems usually use CONFIG_PM_SLEEP_SMP, which sets CONFIG_HOTPLUG_CPU.

I read usually as *optional*

> (I guess the reason why CONFIG_SUSPEND doesn't depend on HOTPLUG_CPU is
> because suspend is possible even on uniprocessor systems and hence the
> Kconfig dependency wasn't really justified).

Again the same question, how do we suspend when HOTPLUG is disabled?
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