Clocksource with rating 0 marked unstable twice

[Josh Hunt]

I have a system with TSCs which are known to be unsynchronized.
During TSC initialization it is marked as unstable, but later on in
the boot the clocksource watchdog code also marks it unstable.  This
seems a bit redundant as both set the rating to 0.

<dmesg snippet>
[    0.000000] Marking TSC unstable due to TSCs unsynchronized
...
[    4.740022] Clocksource tsc unstable (delta = 4380394573887 ns)
</dmesg snippet>

I can think of two solutions, both modifying clocksource_register
when rating is 0:

1. Do not call clocksource_check_watchdog. This way the clocksource
is not put on the watchdog list.

2. Do not enqueue the clocksource or do any registering.  Is there a
good reason to keep around a clocksource which has a rating of 0?

Which of these would be better?  I can provide a patch for either.

Thanks
Josh
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