linux-kernel - [tip:x86/timers] x86/tsc: Set TSC_KNOWN_FREQ and TSC

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Date:   Fri, 18 Nov 2016 02:06:29 -0800
From:   tip-bot for Bin Gao <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     tglx@...utronix.de, linux-kernel@...r.kernel.org,
        bin.gao@...el.com, hpa@...or.com, bin.gao@...ux.intel.com,
        peterz@...radead.org, mingo@...nel.org
Subject: [tip:x86/timers] x86/tsc: Set TSC_KNOWN_FREQ and TSC_RELIABLE flags
 on Intel Atom SoCs

Commit-ID:  f3a02ecebed7df7d5d68898628dea7a3bfcf03e3
Gitweb:     http://git.kernel.org/tip/f3a02ecebed7df7d5d68898628dea7a3bfcf03e3
Author:     Bin Gao <bin.gao@...ux.intel.com>
AuthorDate: Tue, 15 Nov 2016 12:27:24 -0800
Committer:  Thomas Gleixner <tglx@...utronix.de>
CommitDate: Fri, 18 Nov 2016 10:58:31 +0100

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 (provided by HW).

On these platforms PIT/HPET is generally not available so calibration won't
work at all and there is no other clocksource to act as a watchdog for the
TSC, so we have no other choice than to trust it.

Set both X86_FEATURE_TSC_KNOWN_FREQ and X86_FEATURE_TSC_RELIABLE flags to
make sure the calibration is skipped and no watchdog is required.

Signed-off-by: Bin Gao <bin.gao@...el.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Link: http://lkml.kernel.org/r/1479241644-234277-5-git-send-email-bin.gao@linux.intel.com
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>

---
 arch/x86/kernel/tsc_msr.c           | 19 +++++++++++++++++++
 arch/x86/platform/intel-mid/mfld.c  |  9 +++++++--
 arch/x86/platform/intel-mid/mrfld.c |  8 ++++++--
 3 files changed, 32 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/tsc_msr.c b/arch/x86/kernel/tsc_msr.c
index 0fe720d..19afdbd 100644
--- a/arch/x86/kernel/tsc_msr.c
+++ b/arch/x86/kernel/tsc_msr.c
@@ -100,5 +100,24 @@ 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 way for hardware to tell whether the
+	 * 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 mark TSC reliable which removes the
+	 * requirement for a watchdog clocksource.
+	 */
+	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;
 }