linux-kernel - [PATCH 4/4] x86/tsc: set TSC_KNOWN_FREQ and TSC

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Date:   Tue, 15 Nov 2016 12:27:24 -0800
From:   Bin Gao <bin.gao@...ux.intel.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Ingo Molnar <mingo@...hat.com>, H Peter Anvin <hpa@...or.com>,
        x86@...nel.org, Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, Bin Gao <bin.gao@...el.com>
Subject: [PATCH 4/4] x86/tsc: set TSC_KNOWN_FREQ and TSC_RELIABLE flags on Intel Atom SoCs

TSC on Intel Atom SoCs capable of determining TSC frequency by MSR is
reliable and the frequency is known (because it's provided by HW).
On these platforms PIT/HPET is generally not available so
calibration won't work at all and also TSC is the only reliable
clocksource. So we set both X86_FEATURE_TSC_KNOWN_FREQ and
X86_FEATURE_TSC_RELIABLE flags to make sure the calibration is
skipped and no watchdog on TSC.

Signed-off-by: Bin Gao <bin.gao@...el.com>
---
 arch/x86/kernel/tsc_msr.c           | 18 ++++++++++++++++++
 arch/x86/platform/intel-mid/mfld.c  |  9 +++++++--
 arch/x86/platform/intel-mid/mrfld.c |  8 ++++++--
 3 files changed, 31 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/tsc_msr.c b/arch/x86/kernel/tsc_msr.c
index 0fe720d..c0f137c 100644
--- a/arch/x86/kernel/tsc_msr.c
+++ b/arch/x86/kernel/tsc_msr.c
@@ -100,5 +100,23 @@ unsigned long cpu_khz_from_msr(void)
 #ifdef CONFIG_X86_LOCAL_APIC
 	lapic_timer_frequency = (freq * 1000) / HZ;
 #endif
+
+	/*
+	 * TSC frequency determined by MSR is always considered "known"
+	 * because it is reported by HW.
+	 * Another fact is that on MSR capable platforms, PIT/HPET is
+	 * generally not available so calibration won't work at all.
+	 */
+	setup_force_cpu_cap(X86_FEATURE_TSC_KNOWN_FREQ);
+
+	/*
+	 * Unfortunately there is no a HW way to report TSC is reliable.
+	 * We were told by silicon design team that TSC on Atom SoCs are
+	 * always "reliable". TSC is also the only reliable clocksource
+	 * on these SoCs (HPET is either not present or not functional)
+	 * so marke TSC reliable to avoid watchdog on it.
+	 */
+	setup_force_cpu_cap(X86_FEATURE_TSC_RELIABLE);
+
 	return res;
 }
diff --git a/arch/x86/platform/intel-mid/mfld.c b/arch/x86/platform/intel-mid/mfld.c
index 1eb47b6..e793fe5 100644
--- a/arch/x86/platform/intel-mid/mfld.c
+++ b/arch/x86/platform/intel-mid/mfld.c
@@ -49,8 +49,13 @@ static unsigned long __init mfld_calibrate_tsc(void)
 	fast_calibrate = ratio * fsb;
 	pr_debug("read penwell tsc %lu khz\n", fast_calibrate);
 	lapic_timer_frequency = fsb * 1000 / HZ;
-	/* mark tsc clocksource as reliable */
-	set_cpu_cap(&boot_cpu_data, X86_FEATURE_TSC_RELIABLE);
+
+	/*
+	 * TSC on Intel Atom SoCs is reliable and of known frequency.
+	 * See tsc_msr.c for details.
+	 */
+	setup_force_cpu_cap(X86_FEATURE_TSC_KNOWN_FREQ);
+	setup_force_cpu_cap(X86_FEATURE_TSC_RELIABLE);
 
 	return fast_calibrate;
 }
diff --git a/arch/x86/platform/intel-mid/mrfld.c b/arch/x86/platform/intel-mid/mrfld.c
index 59253db..e0607c7 100644
--- a/arch/x86/platform/intel-mid/mrfld.c
+++ b/arch/x86/platform/intel-mid/mrfld.c
@@ -78,8 +78,12 @@ static unsigned long __init tangier_calibrate_tsc(void)
 	pr_debug("Setting lapic_timer_frequency = %d\n",
 			lapic_timer_frequency);
 
-	/* mark tsc clocksource as reliable */
-	set_cpu_cap(&boot_cpu_data, X86_FEATURE_TSC_RELIABLE);
+	/*
+	 * TSC on Intel Atom SoCs is reliable and of known frequency.
+	 * See tsc_msr.c for details.
+	 */
+	setup_force_cpu_cap(X86_FEATURE_TSC_KNOWN_FREQ);
+	setup_force_cpu_cap(X86_FEATURE_TSC_RELIABLE);
 
 	return fast_calibrate;
 }
-- 
1.9.1