intel_pstate: Correct rounding in busy calculation

commit 51d211e9c334b9eca3505f4052afa660c3e0606b upstream.

There was a mistake in the actual rounding portion this previous patch:
f0fe3cd7e12d (intel_pstate: Correct rounding in busy calculation) such that
the rounding was asymetric and incorrect.

Severity: Not very serious, but can increase target pstate by one extra value.
For real world work flows the issue should self correct (but I have no proof).
It is the equivalent of different PID gains for positive and negative numbers.

Examples:
 -3.000000 used to round to -4, rounds to -3 with this patch.
 -3.503906 used to round to -5, rounds to -4 with this patch.

Fixes: f0fe3cd7e12d (intel_pstate: Correct rounding in busy calculation)
Signed-off-by: Doug Smythies <dsmythies@telus.net>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/cpufreq/intel_pstate.c b/drivers/cpufreq/intel_pstate.c
index de9ef4a..6d98c37 100644 (file)
--- a/drivers/cpufreq/intel_pstate.c
+++ b/drivers/cpufreq/intel_pstate.c
@@ -200,10 +200,7 @@ static signed int pid_calc(struct _pid *pid, int32_t busy)
        pid->last_err = fp_error;
 
        result = pterm + mul_fp(pid->integral, pid->i_gain) + dterm;
-       if (result >= 0)
-               result = result + (1 << (FRAC_BITS-1));
-       else
-               result = result - (1 << (FRAC_BITS-1));
+       result = result + (1 << (FRAC_BITS-1));
        return (signed int)fp_toint(result);
 }