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intel_pstate: Add a few comments

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Add a few comments in the code which calculates busyness to
clarify parts of the algorithm.

Signed-off-by: Kristen Carlson Accardi <kristen@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
This commit is contained in:
Kristen Carlson Accardi 2014-12-10 12:39:38 -08:00 committed by Rafael J. Wysocki
parent aa4ea34da9
commit e0d4c8f808
1 changed files with 31 additions and 1 deletions
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32
drivers/cpufreq/intel_pstate.c
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@ -199,7 +199,14 @@ static signed int pid_calc(struct _pid *pid, int32_t busy)
pid->integral += fp_error;
/* limit the integral term */
/*
* We limit the integral here so that it will never
* get higher than 30. This prevents it from becoming
* too large an input over long periods of time and allows
* it to get factored out sooner.
*
* The value of 30 was chosen through experimentation.
*/
integral_limit = int_tofp(30);
if (pid->integral > integral_limit)
pid->integral = integral_limit;
@ -616,6 +623,11 @@ static void intel_pstate_get_min_max(struct cpudata *cpu, int *min, int *max)
if (limits.no_turbo || limits.turbo_disabled)
max_perf = cpu->pstate.max_pstate;
/*
* performance can be limited by user through sysfs, by cpufreq
* policy, or by cpu specific default values determined through
* experimentation.
*/
max_perf_adj = fp_toint(mul_fp(int_tofp(max_perf), limits.max_perf));
*max = clamp_t(int, max_perf_adj,
cpu->pstate.min_pstate, cpu->pstate.turbo_pstate);
@ -717,11 +729,29 @@ static inline int32_t intel_pstate_get_scaled_busy(struct cpudata *cpu)
u32 duration_us;
u32 sample_time;
/*
* core_busy is the ratio of actual performance to max
* max_pstate is the max non turbo pstate available
* current_pstate was the pstate that was requested during
* the last sample period.
*
* We normalize core_busy, which was our actual percent
* performance to what we requested during the last sample
* period. The result will be a percentage of busy at a
* specified pstate.
*/
core_busy = cpu->sample.core_pct_busy;
max_pstate = int_tofp(cpu->pstate.max_pstate);
current_pstate = int_tofp(cpu->pstate.current_pstate);
core_busy = mul_fp(core_busy, div_fp(max_pstate, current_pstate));
/*
* Since we have a deferred timer, it will not fire unless
* we are in C0. So, determine if the actual elapsed time
* is significantly greater (3x) than our sample interval. If it
* is, then we were idle for a long enough period of time
* to adjust our busyness.
*/
sample_time = pid_params.sample_rate_ms * USEC_PER_MSEC;
duration_us = (u32) ktime_us_delta(cpu->sample.time,
cpu->last_sample_time);