intel_pstate: Add a few comments

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>

1 files changed, 31 insertions, 1 deletions

diff --git a/drivers/cpufreq/intel_pstate.c b/drivers/cpufreq/intel_pstate.c
index 0e841eecb743..742eefba12c2 100644
--- a/drivers/cpufreq/intel_pstate.c
+++ b/drivers/cpufreq/intel_pstate.c
@@ -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);