Re: [PATCH 1/3] clocksource: clean up clocksource_select
From: Thomas Gleixner
Date: Thu Jul 04 2013 - 06:27:31 EST
On Thu, 4 Jul 2013, Alex Shi wrote:
>
> We need this check and it is completely unrelated to the problem
>
> you're trying to solve.
>
>
>
> Assume the following:
>
>
>
> System boots with clocksource A and switches into highres mode.
>
> Now clocksource B gets registered and B is not highres capable.
>
>
>
> clocksource_find_best() selects again A, but we have
>
> clocksource=B on the kernel command line to override the kernel
>
> decision.
>
>
>
> By removing the check, you install he non highres capable clocksource
>
> B and kill the machine.
>
>
>
>
You'r right. my bad, Sorry!
>
>
BTW, why we allow user override a second best clocksource? I mean user
>
can override the tsc with hpet. because undetected unstable tsc?
The user can decide to override with clocksource=jiffies if he wants
for testing purposes. That wont switch into highres ever if done from
the kernel command line. Now we have a sysfs interface, so we need to
sanity check the user override against highres.
The override is quite useful to test hpet, pmtimer on a machine which
would always select TSC.
Thanks,
tglx
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