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

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Date:	Mon, 25 Mar 2013 15:53:01 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Arnd Bergmann <arnd@...db.de>
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 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.

> 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.

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.

thanks
-john

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