linux-kernel - Re: [PATCH] clocksource/cadence_ttc: Reuse clocksource as sched

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Date:	Fri, 5 Jul 2013 09:12:51 -0700
From:	Sören Brinkmann <soren.brinkmann@...inx.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	John Stultz <john.stultz@...aro.org>,
	<linux-kernel@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>,
	Michal Simek <michal.simek@...inx.com>
Subject: Re: [PATCH] clocksource/cadence_ttc: Reuse clocksource as sched_clock

On Fri, Jul 05, 2013 at 06:05:14PM +0200, Thomas Gleixner wrote:
> On Fri, 5 Jul 2013, Sören Brinkmann wrote:
> > On Fri, Jul 05, 2013 at 08:30:47AM +0200, Thomas Gleixner wrote:
> > > On Wed, 3 Jul 2013, Soren Brinkmann wrote:
> > > 
> > > > Reuse the TTC clocksource timer as sched clock, too. Since only a single
> > > > sched clock is supported in Linux, this feature optional and can be
> > > > selected through Kconfig.
> > > 
> > > This changelog doesn't make sense.
> > > 
> > > There can be only one active sched_clock, but that does no mean, that
> > > you cannot have different implementations compiled in.
> > > 
> > > So if you disable this config which sched_clock is your kernel using?
> > Jiffies
> > 
> > > And if you enable it, how is guaranteed that you end up with the ttc
> > > sched_clock as the active one? Just due to initcall ordering?
> > I assumed so. Is there a different mechanism?
> 
> jiffies is the default one. If you setup an explicit sched clock then
> this is used. initcall ordering only matters if you have two possible
> sched clocks which might replace jiffies. The one which gets
> registered last wins.
> 
> So the question is, why you want to disable your sched clock at
> compile time.
The timer drivers I have seen unconditionally register themselves as
sched_clock. There does not seem to be a runtime mechanism to choose the
best one - I might miss it though.
I was thinking about this due to the arm_global_timer driver which has
been dicussed on lkml recently, which seemed to do it this way too. And since
that timer would be an alternative sched_clock for Zynq too, I thought I
follow the same approach for the TTC.

	Sören


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