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

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Message-ID: <20121217182202.GA10368@obsidianresearch.com>
Date:	Mon, 17 Dec 2012 11:22:02 -0700
From:	Jason Gunthorpe <jgunthorpe@...idianresearch.com>
To:	Feng Tang <feng.tang@...el.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 Tue, Dec 18, 2012 at 12:14:33AM +0800, Feng Tang wrote:

> > Sure, but my view on this is that it has nothing to do with
> > read_persistent_clock. If the RTC driver can run with IRQs off is a
> > property of the RTC driver and RTC hardware - it has nothing to do
> > with the platform. ARM platforms will vary on a machine by machine
> > basis. The rtc-mv driver used on my ARM system is perfectly
> > re-entrant, lots of rtc on SOC drivers will be the same.
> > 
> > If this is the only thing keeping you on read_persistent_clock, for
> > real RTCs, then how about a RTC_DEV_SAFE_READ flag (or whatever) in
> > rtc_device.flags?
> > 
> > Reserve read_persistent_clock for things like that very specialized
> > non-RTC ARM counter.
> 
> Yes, these non-RTC device is one reason for keeping read_persistent_clock,
> one other reason I can think of is the CONFIG_RTC_LIB is not always on by
> default for all Archs, and some platforms may chose to disable it on purpose. 
> When CONFIG_RTC_LIB is not set, we need the read_persistent_clock for
> time init/suspend/resume.

I thought your concern was the case where the RTC was turned on and
read_persistent_clock was also turned on. Having a flag in the RTC and
disabling read_persistent_clock for that situation would help you
avoid the double code path to the same hardware.

What is motivating having a RTC but not using RTC lib? Embedded
doesn't seem to the be the case, nearly all the interesting rtcs are
under class rtc....

Jason
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