title: Is clocksource tsc always unstable when the only other clocksource is jiffies?
body: <p>After debuging unstable tsc issue for the BIOS of my company's products, I suspect that tsc may be always unstable when the only other clocksource is jiffies.</p>
  
  <p>I got the error like 
  <code>Clocksource tsc unstable (delta = -531266231 ns).</code> Then the kernel selected jiffies other than tsc. 
  The only two clocksources are tsc and jiffies.
  I tried the Linux kernel 2.6 and 3.2 both with i386 and x64. The kernel said that the CPU actually supported constant tsc and invariant tsc. </p>
  
  <p>After checking the Linux source code, I find that tsc has a flag <code>CLOCKSOURCE_MUST_VERIFY</code> while jiffies does not. I guess that if there are only two clocksources, jiffies and tsc, the jiffies will be the clocksource watchdog.
  However, compared to tsc, jiffies is a very poor clocksource, and therefore I suspect that tsc at this situation will always be "unstable" because there is a bad watchdog to verify it.</p>
  
  <p>I also check some other system with well working tsc, and find they have other clocksources like Hpet or acpi_pm.</p>
  
  <p>Therefore, how can I tell whether the tsc instability results from jiffies or some error elsewhere?</p>