[PATCH] cpufreq: intel_pstate: divide by zero on Knights Landing (KNL)

[Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>]

commit 8e601a9f97a00bab031980de34f9a81891c1f82f ("cpufreq: intel_pstate:
Fix divide by zero on Knights Landing (KNL)") from mainline kernel
Backport to 4.1-stable tree

This is a workaround for KNL platform, where in some cases MPERF counter
will not have updated value before next read of MSR_IA32_MPERF. In this
case divide by zero will occur. This change ignores current sample for
busy calculation in this case.

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
---
 drivers/cpufreq/intel_pstate.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/cpufreq/intel_pstate.c b/drivers/cpufreq/intel_pstate.c
index e8d1699..1ee2ab5 100644
--- a/drivers/cpufreq/intel_pstate.c
+++ b/drivers/cpufreq/intel_pstate.c
@@ -761,6 +761,11 @@ static inline void intel_pstate_sample(struct cpudata *cpu)
 	local_irq_save(flags);
 	rdmsrl(MSR_IA32_APERF, aperf);
 	rdmsrl(MSR_IA32_MPERF, mperf);
+	if (cpu->prev_mperf == mperf) {
+		local_irq_restore(flags);
+		return;
+	}
+
 	local_irq_restore(flags);
 
 	cpu->last_sample_time = cpu->sample.time;
-- 
1.9.3