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

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Message-ID: <20121214031330.GC11276@feng-snb>
Date:	Fri, 14 Dec 2012 11:13:30 +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

On Thu, Dec 13, 2012 at 07:38:26PM -0700, Jason Gunthorpe wrote:
> On Thu, Dec 13, 2012 at 06:00:23PM -0800, John Stultz wrote:
> 
> > So per Jason's related patch, he's made the point that the
> > persistent_clock and RTC class functionality are basically exclusive
> > (well, in his case, he said this with respect to updating the RTC,
> > not reading it - I don't mean to put words in his mouth - Please do
> > correct me here Jason. :).  In other words, we probably should avoid
> > configurations where both the rtc hctosys and persistent_clock
> > interfaces are both active.
> 
> I only studied update_persistent_clock, read_persistent_clock is
> very much different.
> 
> Looking at it, I don't think that update_persistent_clock is in any
> way related to read_persistent_clock..  update_persistent_clock is
> *only* called by NTP, and its *only* purpose is to update the RTC with
> NTP synchronized time. In many configurations it will never even be
> called.
> 
> I think update_persistent_clock is badly named, it should be called
> platform_save_ntp_time_to_rtc(), keep it divorced from
> read_presistent_clock :)
> 
> > 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.

Interesting to know this, thanks for the info. For the x86 desktop
and mobile processors I've used, the read_persistent_clock and rtc
are the same on-board device (always power on), so I see many time
related code are execuated twice, like init/suspend/resume if
HCTOSYS config is enabled, that's why I came up with the patches.

> 
> 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?
> 
> Maybe Feng would be better off adjusting read_persistent_clock to
> return ENODEV in such cases??

For mach's without read_persistent_clock capability, there is already
a weakly defined 

	void __attribute__((weak)) read_persistent_clock(struct timespec *ts)
	{
		ts->tv_sec = 0;
		ts->tv_nsec = 0;
	}
so those machs can simply do nothing, and let time core code to judge it.

> 
> 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.

Good point. Figuring out the kconfig for all arm platforms is very
challenging.

Thanks,
Feng
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