linux-kernel - [PATCH 4.2 103/110] cpufreq: intel_pstate: Fix divide by zero on Knights Landing (KNL)

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Message-Id: <20151106191709.356345736@linuxfoundation.org>
Date:	Fri,  6 Nov 2015 11:19:50 -0800
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	linux-kernel@...r.kernel.org
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	stable@...r.kernel.org,
	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
	Kristen Carlson Accardi <kristen@...ux.intel.com>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
Subject: [PATCH 4.2 103/110] cpufreq: intel_pstate: Fix divide by zero on Knights Landing (KNL)

4.2-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>

commit 8e601a9f97a00bab031980de34f9a81891c1f82f upstream.

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.

Fixes: b34ef932d79a (intel_pstate: Knights Landing support)
Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
Acked-by: Kristen Carlson Accardi <kristen@...ux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 drivers/cpufreq/intel_pstate.c |    5 +++++
 1 file changed, 5 insertions(+)

--- a/drivers/cpufreq/intel_pstate.c
+++ b/drivers/cpufreq/intel_pstate.c
@@ -766,6 +766,11 @@ static inline void intel_pstate_sample(s
 	local_irq_save(flags);
 	rdmsrl(MSR_IA32_APERF, aperf);
 	rdmsrl(MSR_IA32_MPERF, mperf);
+	if (cpu->prev_mperf == mperf) {
+		local_irq_restore(flags);
+		return;
+	}
+
 	tsc = native_read_tsc();
 	local_irq_restore(flags);

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