linux-kernel - Re: [PATCH] timekeeping: Move persistent clock registration code from ARM to kernel

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Date:	Fri, 9 Jan 2015 19:48:42 +0000 (UTC)
From:	Paul Walmsley <paul@...an.com>
To:	Thierry Reding <thierry.reding@...il.com>
cc:	Mark Rutland <mark.rutland@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Anatol Pomozov <anatol.pomozov@...il.com>,
	Stephen Warren <swarren@...dotorg.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Russell King <linux@....linux.org.uk>,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	Tony Lindgren <tony@...mide.com>,
	John Stultz <john.stultz@...aro.org>, marc.zyngier@....com,
	pdeschrijver@...dia.com
Subject: Re: [PATCH] timekeeping: Move persistent clock registration code
 from ARM to kernel

Hi

On Fri, 9 Jan 2015, Thierry Reding wrote:

> On Fri, Jan 09, 2015 at 01:59:07PM +0000, Mark Rutland wrote:
> > On Fri, Jan 09, 2015 at 09:49:14AM +0000, Thierry Reding wrote:
> >
> > > As I understand it the architected timer will be turned off along with
> > > the rest of the CPU complex on Tegra. I'm not sure if that's specific to
> > > Tegra or something that other SoCs may do as well.
> > 
> > That doesn't sound right to me: the architecture specifies that the
> > system counter must be implemented in an always-on power domain.
> > 
> > Note that that only applies to the counter, not the timers (as the
> > comparators can be turned off with the CPUs).
> 
> I'll let Paul comment on this, since I don't know the intimate details.

I believe Mark's comments are accurate.

In Linux terms, when deep CPU power management states are in use, the ARM 
architected "timer" is useful as a clocksource, but useless as a wakeup 
source (a clockevent device).

Regarding clocksources, my personal view is that the SoC timer IP 
blocks should be exposed as a clocksource option if the timer IP blocks 
are present in the hardware.  That written, I would expect that most 
production implementations will use the ARM architected "timer" as the 
clocksource.


- Paul
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