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>
diff --git a/drivers/cpufreq/intel_pstate.c b/drivers/cpufreq/intel_pstate.c
index 0e841ee..742eefb 100644
--- a/drivers/cpufreq/intel_pstate.c
+++ b/drivers/cpufreq/intel_pstate.c
@@ -199,7 +199,14 @@
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 @@
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 @@
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);