linux-kernel - Re: [patch 07/15] clocksource: Provide unbind interface in sysfs

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Date:	Wed, 15 May 2013 11:47:42 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	John Stultz <john.stultz@...aro.org>
cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Magnus Damm <magnus.damm@...il.com>
Subject: Re: [patch 07/15] clocksource: Provide unbind interface in sysfs

On Mon, 29 Apr 2013, John Stultz wrote:
> On 04/25/2013 01:31 PM, Thomas Gleixner wrote:
> > With the module refcount held for the current clocksource there is no
> > way to unload the module.
> > 
> > Provide a sysfs interface which allows to unbind the clocksource. One
> > could argue that the clocksource override could be (ab)used to do so,
> > but the clocksource override cannot be used from the kernel itself,
> > while an unbind function can be used to programmatically check whether
> > a clocksource can be shutdown or not.
> > 
> > The unbind functionality uses the new skip current feature of
> > clocksource_select and verifies that a fallback clocksource has been
> > installed. If the clocksource which should be unbound is the current
> > clocksource and no fallback can be found, unbind returns -EBUSY.
> > 
> > This does not support the unbinding of a clocksource which is used as
> > the watchdog clocksource. No point in fostering crappy hardware.
> 
> So.. if the clocksource you want to unbind is the highest rated continuous
> clocksource that doesn't need a watchdog (basically what's likely to be in-use
> and required to be unbinded), its likely to be selected as the watchdog
> already.
> 
> ie: on a system that has only HPET/ACPI_PM, you can't unbind HPET, since its a
> watchdog.

No. The thing is that I only prevent unbinding if it is used as the
watchdog. In the above HPET/PM scenario both are potential watchdogs,
but w/o a user it's valid to unbind one of them.

What I need to prevent is:

TSC is current clocksource and we only have ACPI_PM as watchdog and
its used. So now you try to unbind ACPI_PM then the TSC would be left
w/o a watchdog instance. That's what I'm preventing. Will reword the
changelog accordingly.

Thanks,

	tglx
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