linux-kernel - Re: [PATCH 33/36] AArch64: Generic timers support

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Message-ID: <50006934.1020809@linaro.org>
Date:	Fri, 13 Jul 2012 11:30:12 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Catalin Marinas <catalin.marinas@....com>
CC:	Arnd Bergmann <arnd@...db.de>, Marc Zyngier <Marc.Zyngier@....com>,
	Linus Walleij <linus.walleij@...aro.org>,
	Deepak Saxena <dsaxena@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Walleij <linus.ml.walleij@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Will Deacon <Will.Deacon@....com>,
	Mike Turquette <mike.turquette@...aro.org>,
	Marc Zyngier <maz@...terjones.org>
Subject: Re: [PATCH 33/36] AArch64: Generic timers support

On 07/13/2012 09:02 AM, Catalin Marinas wrote:
> On Fri, Jul 13, 2012 at 01:40:03PM +0100, Arnd Bergmann wrote:
>> Hmm, in the quest to eliminate CLOCK_TICK_RATE entirely, could we
>> make a Kconfig symbol that is selected for all architectures that
>> (may) rely on a periodic timer tick and require this to be set?
>>
>> Architectures that always have a working clock source would then just
>> not include the timex.h header and #define ACT_HZ HZ in common code.
>
> Would something like below be enough?

Looks interesting. Although I've not had a chance to follow through in 
more detail to see what that might impact.  But something like this 
seems like a good approach.


> Also, are there any implications if we boot with clocksource=jiffies?

You'd probably see an maximum error of:
	 (tick-device-granularity/2)/(1sec/HZ).

If that ends up larger then 100ppm you'd have a fairly poor clock, 
although NTP can correct for up to 500ppm.

thanks
-john

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