linux-kernel - Re: [PATCH V4 1/2] cpufreq: Handle sorted frequency tables more efficiently

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Message-ID: <20160626103840.GA32206@ubuntu>
Date:	Sun, 26 Jun 2016 16:08:40 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:	linaro-kernel@...ts.linaro.org, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org, steve.muckle@...aro.org
Subject: Re: [PATCH V4 1/2] cpufreq: Handle sorted frequency tables more
 efficiently

Hi Rafael,

Thanks for having a look at this..

On 23-06-16, 02:28, Rafael J. Wysocki wrote:
> On Tuesday, June 07, 2016 03:55:14 PM Viresh Kumar wrote:
> > +/* Find lowest freq at or above target in a table in ascending order */
> > +static inline int cpufreq_table_find_index_al(struct cpufreq_policy *policy,
> > +					      unsigned int target_freq)
> > +{
> > +	struct cpufreq_frequency_table *table = policy->freq_table;
> > +	unsigned int freq;
> > +	int i, best = -1;
> > +
> > +	for (i = 0; table[i].frequency != CPUFREQ_TABLE_END; i++) {
> > +		freq = table[i].frequency;
> > +
> > +		if (freq_is_invalid(policy, freq))
> > +			continue;
> > +
> > +		if (freq >= target_freq)
> > +			return i;
> > +
> > +		best = i;
> > +	}
> > +
> > +	if (best == -1) {
> > +		WARN(1, "Invalid frequency table: %d\n", policy->cpu);
> 
> After a successful cpufreq_table_validate_and_show() that should be impossible,
> shouldn't it?

This shouldn't be possible unless cpufreq_table_validate_and_show() has a bug,
or somehow that routine isn't called.

Though, to catch such bugs, what about WARN_ON(best == -1); ? The WARN() will
have an unlikely() statement as well to optimize it and we can catch the bugs as
well.

Or if you think we should just remove them..

> > +/* Find highest freq at or below target in a table in descending order */
> > +static inline int cpufreq_table_find_index_dh(struct cpufreq_policy *policy,
> > +					      unsigned int target_freq)
> > +{
> > +	struct cpufreq_frequency_table *table = policy->freq_table;
> > +	unsigned int freq;
> > +	int i, best = -1;
> > +
> > +	for (i = 0; table[i].frequency != CPUFREQ_TABLE_END; i++) {
> > +		freq = table[i].frequency;
> > +
> > +		if (freq_is_invalid(policy, freq))
> > +			continue;
> > +
> > +		if (freq <= target_freq)
> > +			return i;
> > +
> > +		best = i;
> > +	}
> > +
> > +	if (best == -1) {
> > +		WARN(1, "Invalid frequency table: %d\n", policy->cpu);
> > +		return -EINVAL;
> > +	}
> > +
> > +	return best;
> > +}
> 
> I still don't see a reason for min/max checking in these routines.
> 
> So what is the reason?

These routines are all part of the existing API cpufreq_frequency_table_target()
and that always had these checks. Over that, not all of its callers are ensuring
that the target-freq is clamped before this routine is called. And so we need to
make sure that these routines return a frequency between min/max only.

What do you say ?

-- 
viresh