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    SubjectRe: [PATCH v1 3/4] cpufreq: Add special-purpose fast-switching callback for drivers
    On 07-12-20, 17:35, Rafael J. Wysocki wrote:
    > From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
    >
    > First off, some cpufreq drivers (eg. intel_pstate) can pass hints
    > beyond the current target frequency to the hardware and there are no
    > provisions for doing that in the cpufreq framework. In particular,
    > today the driver has to assume that it should not allow the frequency
    > to fall below the one requested by the governor (or the required
    > capacity may not be provided) which may not be the case and which may
    > lead to excessive energy usage in some scenarios.
    >
    > Second, the hints passed by these drivers to the hardware need not be
    > in terms of the frequency, so representing the utilization numbers
    > coming from the scheduler as frequency before passing them to those
    > drivers is not really useful.
    >
    > Address the two points above by adding a special-purpose replacement
    > for the ->fast_switch callback, called ->adjust_perf, allowing the
    > governor to pass abstract performance level (rather than frequency)
    > values for the minimum (required) and target (desired) performance
    > along with the CPU capacity to compare them to.
    >
    > Also update the schedutil governor to use the new callback instead
    > of ->fast_switch if present.
    >
    > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
    > ---
    >
    > Changes with respect to the RFC:
    > - Don't pass "busy" to ->adjust_perf().
    > - Use a special 'update_util' hook for the ->adjust_perf() case in
    > schedutil (this still requires an additional branch because of the
    > shared common code between this case and the "frequency" one, but
    > IMV this version is cleaner nevertheless).
    >
    > ---
    > drivers/cpufreq/cpufreq.c | 40 ++++++++++++++++++++++++++++++++
    > include/linux/cpufreq.h | 14 +++++++++++
    > include/linux/sched/cpufreq.h | 5 ++++
    > kernel/sched/cpufreq_schedutil.c | 48 +++++++++++++++++++++++++++++++--------
    > 4 files changed, 98 insertions(+), 9 deletions(-)
    >
    > Index: linux-pm/include/linux/cpufreq.h
    > ===================================================================
    > --- linux-pm.orig/include/linux/cpufreq.h
    > +++ linux-pm/include/linux/cpufreq.h
    > @@ -320,6 +320,15 @@ struct cpufreq_driver {
    > unsigned int index);
    > unsigned int (*fast_switch)(struct cpufreq_policy *policy,
    > unsigned int target_freq);
    > + /*
    > + * ->fast_switch() replacement for drivers that use an internal
    > + * representation of performance levels and can pass hints other than
    > + * the target performance level to the hardware.
    > + */
    > + void (*adjust_perf)(unsigned int cpu,
    > + unsigned long min_perf,
    > + unsigned long target_perf,
    > + unsigned long capacity);

    With this callback in place, do we still need to keep the other stuff we
    introduced recently, like CPUFREQ_NEED_UPDATE_LIMITS ?

    --
    viresh

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