x86/tsc: Future-proof native_calibrate_tsc()

BugLink: http://bugs.launchpad.net/bugs/1744988
If the crystal frequency cannot be determined via CPUID(15).crystal_khz or
the built-in table then native_calibrate_tsc() will still set the
X86_FEATURE_TSC_KNOWN_FREQ flag which prevents the refined TSC calibration.

As a consequence such systems use cpu_khz for the TSC frequency which is
incorrect when cpu_khz != tsc_khz resulting in time drift.

Return early when the crystal frequency cannot be retrieved without setting
the X86_FEATURE_TSC_KNOWN_FREQ flag. This ensures that the refined TSC
calibration is invoked.

[ tglx: Steam-blastered changelog. Sigh ]

Fixes: 4ca4df0b7eb0 ("x86/tsc: Mark TSC frequency determined by CPUID as known")
Signed-off-by: Len Brown <len.brown@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: peterz@infradead.org
Cc: Bin Gao <bin.gao@intel.com>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/0fe2503aa7d7fc69137141fc705541a78101d2b9.1513920414.git.len.brown@intel.com
(cherry picked from commit da4ae6c4a0b8dee5a5377a385545d2250fa8cddb)
Signed-off-by: Joseph Salisbury <joseph.salisbury@canonical.com>
Acked-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Khalid Elmously <khalid.elmously@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 531cc9306c93f3f1eaae7c927bc2be8e7f34655f..8f5cf85576258531e40dd42d3312753de7f6402e 100644 (file)
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -612,6 +612,8 @@ unsigned long native_calibrate_tsc(void)
                }
        }
 
+       if (crystal_khz == 0)
+               return 0;
        /*
         * TSC frequency determined by CPUID is a "hardware reported"
         * frequency and is the most accurate one so far we have. This