linux-kernel - Re: [RFC][PATCH 1/4] RTC: Class device support for persistent clock

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Date:	Sun, 18 May 2008 05:39:47 +0100 (BST)
From:	"Maciej W. Rozycki" <macro@...ux-mips.org>
To:	john stultz <johnstul@...ibm.com>
cc:	Alessandro Zummo <a.zummo@...ertech.it>,
	Jean Delvare <khali@...ux-fr.org>,
	Ralf Baechle <ralf@...ux-mips.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	rtc-linux@...glegroups.com, i2c@...sensors.org,
	linux-mips@...ux-mips.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 1/4] RTC: Class device support for persistent clock

Hi John,

 Sorry about the delay -- I have missed your comment in the flood.

On Wed, 7 May 2008, john stultz wrote:

> >   As rtc_read_persistent_clock() is not available at the time
> >  timekeeping_init() is called, it will now be disabled if the class device
> >  is to be used as a reference.  In this case rtc_hctosys(), already
> >  present, will be used to set up the system time at the late initcall time.
> >  This call has now been rewritten to make use of
> >  rtc_read_persistent_clock().
> 
> Hrmm. So how is this going to work with suspend and resume?

 Hmm, I have never used suspend/resume, so I cannot really comment.  Here
is what I gathered by glancing over the code and some bits of
documentation.

> Ideally, on resume we want to update the clock before interrupts are
> reenabled so we don't get stale time values post-resume.  For systems
> that sleep on reading the persistent clock, I'm open to having them
> fix it up as best they can later (partly why the code can handle
> read_persistent_clock() not returning anything), but unless I'm
> misreading this, it seems you're proposing to make systems that do
> have a safe persistent clock have to have the window where code may
> see the pre-suspend time after resume.

 Right now it looks the time is restored in two places, 
timekeeping_resume() and rtc_resume().  Of course once the transition to 
the new RTC infrastructure has been done, one is going to be redundant.  
For the time being I think it is harmless to have them both.

 That written, both are called from the relevant driver's ->resume()  
method.  My set of patches does not change it and as far as I can tell if
it worked before, it will work afterwards.  As I understand ->resume()  
methods may sleep and are called with interrupts already enabled.

> Am I missing something here?

 No idea -- anyone?

  Maciej
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