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Date: Mon, 30 May 2016 09:20:27 -0700
From: Steve Muckle <steve.muckle@...aro.org>
To: Viresh Kumar <viresh.kumar@...aro.org>
Cc: Steve Muckle <steve.muckle@...aro.org>,
Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>,
"Rafael J. Wysocki" <rafael@...nel.org>,
linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
Vincent Guittot <vincent.guittot@...aro.org>,
Morten Rasmussen <morten.rasmussen@....com>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Juri Lelli <Juri.Lelli@....com>,
Patrick Bellasi <patrick.bellasi@....com>,
Michael Turquette <mturquette@...libre.com>
Subject: Re: [PATCH v2 2/3] cpufreq: acpi-cpufreq: add resolve_freq callback
On Thu, May 26, 2016 at 12:13:41PM +0530, Viresh Kumar wrote:
> On 25-05-16, 19:53, Steve Muckle wrote:
> > Support the new resolve_freq cpufreq callback which resolves a target
> > frequency to a driver-supported frequency without actually setting it.
>
> And here is the first abuser of this API as I was talking about in the
> earlier patch :)
>
> But, I know why you are doing it and I think we can do it differently.
>
> So, lets assume that the ->resolve_freq() callback will ONLY be
> provided by the drivers which also provide a ->target() callback.
>
> i.e. not by acpi-cpufreq at least.
>
> > The target frequency and resolved frequency table entry are cached so
> > that a subsequent fast_switch operation may avoid the frequency table
> > walk assuming the requested target frequency is the same.
> >
> > Suggested-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > Signed-off-by: Steve Muckle <smuckle@...aro.org>
> > ---
> > drivers/cpufreq/acpi-cpufreq.c | 56 ++++++++++++++++++++++++++++++++----------
> > 1 file changed, 43 insertions(+), 13 deletions(-)
> >
> > diff --git a/drivers/cpufreq/acpi-cpufreq.c b/drivers/cpufreq/acpi-cpufreq.c
> > index 7f38fb55f223..d87962eda1ed 100644
> > --- a/drivers/cpufreq/acpi-cpufreq.c
> > +++ b/drivers/cpufreq/acpi-cpufreq.c
> > @@ -66,6 +66,8 @@ enum {
> >
> > struct acpi_cpufreq_data {
> > struct cpufreq_frequency_table *freq_table;
> > + unsigned int cached_lookup_freq;
> > + struct cpufreq_frequency_table *cached_lookup_entry;
>
> This could have been an integer value 'Index', which could have been
> used together with the freq-table to get the freq we wanted.
>
> And, then we can move these two fields into the cpufreq_policy
> structure and make them part of the first patch itself.
>
> > unsigned int resume;
> > unsigned int cpu_feature;
> > unsigned int acpi_perf_cpu;
> > @@ -458,26 +460,53 @@ static int acpi_cpufreq_target(struct cpufreq_policy *policy,
> > return result;
> > }
> >
> > -unsigned int acpi_cpufreq_fast_switch(struct cpufreq_policy *policy,
> > - unsigned int target_freq)
> > +/*
> > + * Find the closest frequency above target_freq.
> > + *
> > + * The table is sorted in the reverse order with respect to the
> > + * frequency and all of the entries are valid (see the initialization).
> > + */
> > +static inline struct cpufreq_frequency_table
> > +*lookup_freq(struct cpufreq_frequency_table *table, unsigned int target_freq)
> > {
> > - struct acpi_cpufreq_data *data = policy->driver_data;
> > - struct acpi_processor_performance *perf;
> > - struct cpufreq_frequency_table *entry;
> > - unsigned int next_perf_state, next_freq, freq;
> > + struct cpufreq_frequency_table *entry = table;
> > + unsigned int freq;
> >
> > - /*
> > - * Find the closest frequency above target_freq.
> > - *
> > - * The table is sorted in the reverse order with respect to the
> > - * frequency and all of the entries are valid (see the initialization).
> > - */
> > - entry = data->freq_table;
> > do {
> > entry++;
> > freq = entry->frequency;
> > } while (freq >= target_freq && freq != CPUFREQ_TABLE_END);
> > entry--;
> > +
> > + return entry;
> > +}
> > +
> > +unsigned int acpi_cpufreq_resolve_freq(struct cpufreq_policy *policy,
> > + unsigned int target_freq)
> > +{
> > + struct acpi_cpufreq_data *data = policy->driver_data;
> > + struct cpufreq_frequency_table *entry;
> > +
> > + data->cached_lookup_freq = target_freq;
> > + entry = lookup_freq(data->freq_table, target_freq);
> > + data->cached_lookup_entry = entry;
> > +
> > + return entry->frequency;
> > +}
> > +
>
> And then we could remove this callback from acpi-cpufreq.
>
> > +unsigned int acpi_cpufreq_fast_switch(struct cpufreq_policy *policy,
> > + unsigned int target_freq)
> > +{
> > + struct acpi_cpufreq_data *data = policy->driver_data;
> > + struct acpi_processor_performance *perf;
> > + struct cpufreq_frequency_table *entry;
> > + unsigned int next_perf_state, next_freq;
> > +
> > + if (data->cached_lookup_entry &&
> > + data->cached_lookup_freq == target_freq)
> > + entry = data->cached_lookup_entry;
> > + else
> > + entry = lookup_freq(data->freq_table, target_freq);
>
> And a static inline callback in cpufreq.h to get this entry.
>
> > next_freq = entry->frequency;
> > next_perf_state = entry->driver_data;
> >
> > @@ -918,6 +947,7 @@ static struct cpufreq_driver acpi_cpufreq_driver = {
> > .verify = cpufreq_generic_frequency_table_verify,
> > .target_index = acpi_cpufreq_target,
> > .fast_switch = acpi_cpufreq_fast_switch,
> > + .resolve_freq = acpi_cpufreq_resolve_freq,
> > .bios_limit = acpi_processor_get_bios_limit,
> > .init = acpi_cpufreq_cpu_init,
> > .exit = acpi_cpufreq_cpu_exit,
>
> Sounds reasonable ?
A couple concerns... One is that if we do the lookup in
cpufreq_driver_resolve_freq() for drivers which implement target_index()
then it means using cpufreq_frequency_table_target() there. This is a
heavier weight function that can't take advantage of driver-specific
knowledge that the freq table is sorted a particular way. So for
acpi-cpufreq we'd now be having to walk the whole table for every
fast_switch.
Another is that it'll be a a bit odd that the logic used to lookup the
driver frequency will be different in the cached and uncached
fast_switch cases. In the cached case it will have been determined by
code in cpufreq_driver_resolve_freq() whereas in the uncached case it
will be logic in the driver, in its fast_switch routine.
I think at least the first issue would need to be solved to use this
approach as it would impact performance for every fast_switch call.
(Thanks for the review btw!)
thanks,
Steve
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