intel_pstate: Correct rounding in busy calculation

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>
Cc: 3.14+ <stable@vger.kernel.org> # 3.14+
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 4e7f492ad5839d43ef02a092d1b8cd9fde735278..924bb2d42b1c8cd4121289f1569107ffa3a58a4b 100644 (file)
--- a/drivers/cpufreq/intel_pstate.c
+++ b/drivers/cpufreq/intel_pstate.c
@@ -196,10 +196,7 @@ 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);
 }