x86/tsc: Fix ART for TSC_KNOWN_FREQ

[Peter Zijlstra <peterz@infradead.org>]

Subhransu reported that convert_art_to_tsc() isn't working for him.

It turns out that because of commit:

  57779dc2b3b7 ("x86, tsc: Skip refined tsc calibration on systems with reliable TSC")

Systems with TSC_KNOWN_FREQ (due to having CPUID.15h) will not run the
refined calibration thing, and then forget to set the ART clocksource
relation.

We cannot set the ART relation prior to running the refined calibration;
therefore duplicate the lines.

XXX: should we clear this in mark_tsc_unstable() ?

Reported-by: "Prusty, Subhransu S" <subhransu.s.prusty@intel.com>
Cc: kevin.b.stanton@intel.com
Cc: Alok N Kataria <akataria@vmware.com>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Christopher Hall <christopher.s.hall@intel.com>
Fixes: 57779dc2b3b7 ("x86, tsc: Skip refined tsc calibration on systems with reliable TSC")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 arch/x86/kernel/tsc.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 4f7a9833d8e5..c73a7f9e881a 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -1333,6 +1333,8 @@ static int __init init_tsc_clocksource(void)
 	 * the refined calibration and directly register it as a clocksource.
 	 */
 	if (boot_cpu_has(X86_FEATURE_TSC_KNOWN_FREQ)) {
+		if (boot_cpu_has(X86_FEATURE_ART))
+			art_related_clocksource = &clocksource_tsc;
 		clocksource_register_khz(&clocksource_tsc, tsc_khz);
 		return 0;
 	}