linux-kernel - Re: [PATCH 7/8] x86/mrst: add vrtc driver which serves as a wall clock device

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Date:	Tue, 18 May 2010 09:38:32 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Feng Tang <feng.tang@...el.com>
cc:	Jacob Pan <jacob.jun.pan@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
	"Du, Alek" <alek.du@...el.com>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	LKML <linux-kernel@...r.kernel.org>,
	John Stultz <johnstul@...ibm.com>
Subject: Re: [PATCH 7/8] x86/mrst: add vrtc driver which serves as a wall
 clock device

On Tue, 18 May 2010, Feng Tang wrote:
> Hi Thomas,
> 
> 
> On Mon, 17 May 2010 17:15:55 +0800
> Thomas Gleixner <tglx@...utronix.de> wrote:
> 
> 
> > > 
> > > Actually when to init the vrtc register is a big problem for me,
> > > vrtc need be inited before timekeeping_init(), and I thought better
> > > to put it somewhere in setup_arch(), as it is architecture
> > > specific, and ioremap is not working at that time. Also that's the
> > > reason I created a new wallclock_init func for x86_platforms, I
> > > could not find a better way to do the vrtc init.
> > 
> > There is no particular reason why we need to read it in
> > timekeeping_init(). Nothing in the kernel needs the correct wall time
> > at that point. So we can safely move the setting of xtime to rtc wall
> > clock time to a separate timekeeping_late_init() function.
> > 
> > John ???
> >  
> Yeah, good suggestion, if xtime init is moved to a later time in kernel
> init flow, then vrtc's init function can be set a arch_initcall()

John was away yesterday, so we have to wait for his answer, but I don't
expect a nono from him.
 
> > > > > +	lock_cmos_prefix(reg);
> > > > 
> > > >   This lock_cmos magic should just die. I have no idea why
> > > > something wants or wanted to access the RTC from an NMI.
> > > 
> > > I will try to reuse the rtc_lock defined in rtc.c whose get/set_time
> > > service won't be called with vrtc's at the same time.
> > 
> > Please don't create artifical dependencies. Use a separate vrtc_lock
> > to serialize the access to vrtc.
> I just checked the code, when wall clock's get/set_time service is called,
> it is always protected by rtc_lock(code in arch/x86/kernel/rtc.c), then
> no need to add the lock for each individual register read/write operation.

Grr. Yes, missed that the code is called under rtc_lock already. So
you can drop the locking in your code completely.

Thanks,

	tglx
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