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

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Date:	Tue, 22 Feb 2011 13:21:13 -0800
From:	john stultz <johnstul@...ibm.com>
To:	Mark Brown <broonie@...nsource.wolfsonmicro.com>
Cc:	rtc-linux@...glegroups.com, LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Alessandro Zummo <a.zummo@...ertech.it>,
	Marcelo Roberto Jimenez <mroberto@...i.cetuc.puc-rio.br>
Subject: Re: [rtc-linux] [PATCH 04/10] RTC: Cleanup
 rtc_class_ops->read_alarm()

On Tue, 2011-02-22 at 21:05 +0000, Mark Brown wrote:
> On Tue, Feb 22, 2011 at 12:22:54PM -0800, john stultz wrote:
> 
> > In some ways it does complicate things, but in others it greatly
> > simplifies it. You don't have to have 80 drivers each implementing their
> > own code to set a mode that isn't used. Everyone is using the common
> > kernel code, so bugs are shared and thus found and fixed faster.
> > Features can be more easily added, as the limitations of specific
> > hardware have to be more formally expressed, rather then having to
> > change 80 drivers that opaquely work around their specific hardware
> > issues. Also, applications are easier to port, since there are less
> > platform specific differences.
> 
> I agree that it's a win for things like UIE - the reason it worries me
> for alarms (and the RTC time itself) is that full emulation requires us
> to do things over reboots, including the support for having multiple
> alarms scheduled which isn't available on most hardware at all.

Hmm. Maybe I'm missing what you mean again. If the RTC doesn't support
alarms, we don't emulate them (or RTC time).

But if you just mean trying to keep multiple alarms scheduled across
resets, I don't think that is something we can emulate (since the kernel
doesn't have any other persistent storage). But due to the lack of
consistency in RTC hardware, I don't think its a reasonable expectation
for applications to have.

We can preserve what hardware state we can at boot, but applications
should not expect alarms set multiple reboot cycles ago to be valid.
After all, other applications might have jumped in and grabbed the rtc
device and set it to something else before the application was able to.
Or a user might change the value from something like a bios menu.

thanks
-john

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