linux-kernel - Re: [PATCH] ARM: convert arm/arm64 arch timer to use CLKSRC

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Message-Id: <201303260956.28784.arnd@arndb.de>
Date:	Tue, 26 Mar 2013 09:56:28 +0000
From:	Arnd Bergmann <arnd@...db.de>
To:	John Stultz <john.stultz@...aro.org>
Cc:	Rob Herring <robherring2@...il.com>,
	"Russell King - ARM Linux" <linux@....linux.org.uk>,
	Mark Rutland <mark.rutland@....com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Kukjin Kim <kgene.kim@...sung.com>,
	"linux-sh@...r.kernel.org" <linux-sh@...r.kernel.org>,
	Tony Lindgren <tony@...mide.com>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Magnus Damm <magnus.damm@...il.com>,
	"rob.herring@...xeda.com" <rob.herring@...xeda.com>,
	"linux-samsung-soc@...r.kernel.org" 
	<linux-samsung-soc@...r.kernel.org>,
	Simon Horman <horms@...ge.net.au>,
	Will Deacon <Will.Deacon@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>
Subject: Re: [PATCH] ARM: convert arm/arm64 arch timer to use CLKSRC_OF init

On Monday 25 March 2013, John Stultz wrote:
> On 03/25/2013 03:36 PM, Arnd Bergmann wrote:
> > On Monday 25 March 2013, Rob Herring wrote:
> >> I count integrator-cp, realview, versatile and non-DT VExpress that do
> >> this (not surprisingly) and 25 platforms or timer implementations plus
> >> arm64 that do sched_clock setup in time_init. What's broken by not
> >> moving these earlier?
> > timekeeping_init() will leave the persistent_clock_exist variable as "false",
> > which is read in rtc_suspend() and timekeeping_inject_sleeptime().
> 
> Are you mixing up the persistent_clock and sched_clock here? From a 
> generic stand-point they have different requirements.

Ah, sorry about that. I had stumbled over the persistent_clock
issue earlier and was confusing the two.

> > For all I can tell, you will get a little jitter every time you
> > do a suspend in that case. Or perhaps it means the system clock
> > will be forwarded by the amount of time spent in suspend twice
> > after wakeup, but I'm probably misreading the code for that case.
> 
> No, you shouldn't see timekeeping being incremented twice, we check in 
> rtc_resume code if the persistent clock is present if so we won't inject 
> any measured suspend time there. But you're probably right that we're 
> being a little overly paranoid checking the same value twice.

Well, the point is that has_persistent_clock() returns false because
it is not yet active when the flag gets set in timekeeping_init(),
but when we call read_persistent_clock() in timekeeping_suspend(),
it will actually return a non-zero time.
 
> As far as the benefit to the persistent clock: it is just a little 
> better to use, since we can access it earlier in resume, prior to 
> interrupts being enabled. So we should see less time error introduced 
> each suspend.

Ok.

	Arnd
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