x86/tsc: Mark Intel ATOM_GOLDMONT TSC reliable

On Intel GOLDMONT Atom SoC TSC is the only available clocksource, so there
is no way to do software calibration or have a watchdog clocksource for it.
Software calibration is already disabled via the TSC_KNOWN_FREQ flag, but
the watchdog requirement still persists, so such systems cannot switch to
high resolution/nohz mode.

Mark it reliable, so it becomes usable. Hardware teams confirmed that this
is safe on that SoC.

Signed-off-by: Bin Gao <bin.gao@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1479241644-234277-4-git-send-email-bin.gao@linux.intel.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index e58c319..f4dfdaa 100644 (file)
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -709,6 +709,13 @@ unsigned long native_calibrate_tsc(void)
         */
        setup_force_cpu_cap(X86_FEATURE_TSC_KNOWN_FREQ);
 
+       /*
+        * For Atom SoCs TSC is the only reliable clocksource.
+        * Mark TSC reliable so no watchdog on it.
+        */
+       if (boot_cpu_data.x86_model == INTEL_FAM6_ATOM_GOLDMONT)
+               setup_force_cpu_cap(X86_FEATURE_TSC_RELIABLE);
+
        return crystal_khz * ebx_numerator / eax_denominator;
 }