linux-kernel - Re: [PATCH 1/3] timekeeping: Add persistent_clock

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Message-ID: <20121220070255.GB16475@feng-snb>
Date:	Thu, 20 Dec 2012 15:02:55 +0800
From:	Feng Tang <feng.tang@...el.com>
To:	Jason Gunthorpe <jgunthorpe@...idianresearch.com>
Cc:	John Stultz <john.stultz@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Alessandro Zummo <a.zummo@...ertech.it>,
	linux-kernel@...r.kernel.org, alek.du@...el.com
Subject: Re: [PATCH 1/3] timekeeping: Add persistent_clock_exist flag

Hi Jason,

On Thu, Dec 13, 2012 at 07:38:26PM -0700, Jason Gunthorpe wrote:
> 
> > make the HCTOSYS option be dependent on !HAS_PERSISTENT_CLOCK. This
> > way we avoid having configs where there are conflicting paths that
> > we chose from.
> 
> On ARM the read_presistent_clock is used to access a true monotonic
> counter that is divorced from the system RTC - look at
> arch/arm/plat-omap/counter_32k.c for instance.
> 
> This seems like a great use of that hardware resource, and no doubt
> those mach's also have a class RTC driver available talking to
> different hardware.
> 
> For mach's without that functionality ARM returns a fixed 0 value
> from read_persistent_clock, persumably the kernel detects this and
> falls back to using class rtc functions?

I read the counter_32k.c and the read_persistent_clock() magic for
ARM, and got a question: for the omap platforms using the
counter_32k.c's read_persistent_clock, why will they also calles
the time service from RTC device like the rtc_suspend/resume? I 
thought usually we just need to use the time source with the better
accuracy.

Thanks,
Feng

> 
> Maybe Feng would be better off adjusting read_persistent_clock to
> return ENODEV in such cases??
> 
> So, I think you have to keep your test as a run time test. To support
> the single image ARM boot you can't make the distinction with kconfig.
> 
> Regards,
> Jason
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