Subject: Re: [PATCH v2 2/6] powernv:cpufreq: Create a powernv_cpu_to_core_mask() helper.

From: "Srivatsa S. Bhat" <[email-masked]>
Date: Wed, 19 Mar 2014 12:05:20 +0530

On 03/19/2014 05:07 AM, Benjamin Herrenschmidt wrote:
> On Mon, 2014-03-10 at 16:40 +0530, Gautham R. Shenoy wrote:
>> From: "Srivatsa S. Bhat" <[email-masked]>
>>
>> Create a helper method that computes the cpumask corresponding to the
>> thread-siblings of a cpu. Use this for initializing the policy->cpus
>> mask for a given cpu.
>>
>> (Original code written by Srivatsa S. Bhat. Gautham moved this to a
>> helper function!)
> 
> How does that differ from the existing entry in the sibling map  which
> you can obtain with the helper cpu_sibling_mask() ? (You probably want
> to *copy* the mask of course but I don't see the need of re-creating
> one from scratch).
>

The intent here was to have a sibling mask that is invariant of CPU
hotplug. cpu_sibling_mask is updated on every hotplug operation, and this
would break cpufreq, since policy->cpus has to be hotplug invariant.

This should have been noted in the changelog of patch 1 as well as this
patch. (The earlier (internal) versions of this patchset had the
description in the changelogs, but somehow it got dropped accidentally).
I'll work with Gautham and ensure that we have this important info
included in the changelog. Thanks for pointing it out!

Regards,
Srivatsa S. Bhat

> Also, this should have been CCed to the cpufreq mailing list and maybe
> the relevant maintainer too.
> 
> Cheers,
> Ben.
> 
>> Signed-off-by: Srivatsa S. Bhat <[email-masked]>
>> Signed-off-by: Gautham R. Shenoy <[email-masked]>
>> ---
>>  drivers/cpufreq/powernv-cpufreq.c | 24 ++++++++++++++++++------
>>  1 file changed, 18 insertions(+), 6 deletions(-)
>>
>> diff --git a/drivers/cpufreq/powernv-cpufreq.c b/drivers/cpufreq/powernv-cpufreq.c
>> index ab1551f..4cad727 100644
>> --- a/drivers/cpufreq/powernv-cpufreq.c
>> +++ b/drivers/cpufreq/powernv-cpufreq.c
>> @@ -115,6 +115,23 @@ static struct freq_attr *powernv_cpu_freq_attr[] = {
>>  
>>  /* Helper routines */
>>  
>> +/**
>> + * Sets the bits corresponding to the thread-siblings of cpu in its core
>> + * in 'cpus'.
>> + */
>> +static void powernv_cpu_to_core_mask(unsigned int cpu, cpumask_var_t cpus)
>> +{
>> +	int base, i;
>> +
>> +	base = cpu_first_thread_sibling(cpu);
>> +
>> +	for (i = 0; i < threads_per_core; i++) {
>> +		cpumask_set_cpu(base + i, cpus);
>> +	}
>> +
>> +	return;
>> +}
>> +
>>  /* Access helpers to power mgt SPR */
>>  
>>  static inline unsigned long get_pmspr(unsigned long sprn)
>> @@ -180,13 +197,8 @@ static int powernv_set_freq(cpumask_var_t cpus, unsigned int new_index)
>>  
>>  static int powernv_cpufreq_cpu_init(struct cpufreq_policy *policy)
>>  {
>> -	int base, i;
>> -
>>  #ifdef CONFIG_SMP
>> -	base = cpu_first_thread_sibling(policy->cpu);
>> -
>> -	for (i = 0; i < threads_per_core; i++)
>> -		cpumask_set_cpu(base + i, policy->cpus);
>> +	powernv_cpu_to_core_mask(policy->cpu, policy->cpus);
>>  #endif
>>  	policy->cpuinfo.transition_latency = 25000;
>>  
> 
>