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

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Message-ID: <20121214023826.GA31613@obsidianresearch.com>
Date:	Thu, 13 Dec 2012 19:38:26 -0700
From:	Jason Gunthorpe <jgunthorpe@...idianresearch.com>
To:	John Stultz <john.stultz@...aro.org>
Cc:	Feng Tang <feng.tang@...el.com>,
	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 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.

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??

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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