linux-kernel - Re: [rtc-linux] [PATCH 04/10] RTC: Cleanup rtc_class_ops->read

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Date:	Tue, 22 Feb 2011 11:58:16 -0800
From:	john stultz <johnstul@...ibm.com>
To:	Mark Brown <broonie@...nsource.wolfsonmicro.com>
Cc:	Marcelo Roberto Jimenez <mroberto@...i.cetuc.puc-rio.br>,
	rtc-linux@...glegroups.com, LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Alessandro Zummo <a.zummo@...ertech.it>
Subject: Re: [rtc-linux] [PATCH 04/10] RTC: Cleanup
 rtc_class_ops->read_alarm()

On Tue, 2011-02-22 at 19:51 +0000, Mark Brown wrote:
> On Tue, Feb 22, 2011 at 11:35:10AM -0800, john stultz wrote:
> 
> > Yea. The way I thought about it originally was that you can set an alarm
> > and that alarm will fire if the machine is on, suspended or even in some
> > cases off.  Then, when the machine is booted (system reset), the state
> > of the RTC's alarm should not be trusted.
> 
> > Your description of the AIE/UIE having random values aligns with that
> > intuition.
> 
> This seems rather worrying - it sounds like it might mean that the
> device might come up firing spuriously which doesn't seem terribly
> clever.

Well, in those known cases the driver should initalize the irq modes to
be off. 

> > However, if the expectation is that once set, the alarm should persist
> > across any number of reboots, this makes it a bit more complicated.
> 
> For an embedded device I'd expect that either nothing about the RTC
> would persist (including the time) or everything would.

But that isn't the reality of the hardware. On reboot the kernel can't
trust hardware to be in a valid state.

Even so, we can try to preserve what we can, but I think the expectation
from an application's point of view shouldn't be that rtc device's alarm
state will be valid upon reset.

thanks
-john


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