linux-kernel - Re: [PATCH 6/9] rtc-pcf2123: avoid resetting the clock if possible

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Message-ID: <20151201210417.GA22136@piout.net>
Date:	Tue, 1 Dec 2015 22:04:17 +0100
From:	Alexandre Belloni <alexandre.belloni@...e-electrons.com>
To:	Joshua Clayton <stillcompiling@...il.com>
Cc:	Alessandro Zummo <a.zummo@...ertech.it>,
	rtc-linux@...glegroups.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6/9] rtc-pcf2123: avoid resetting the clock if possible

On 01/12/2015 at 12:23:51 -0800, Joshua Clayton wrote :
> On Wed, 25 Nov 2015 00:25:12 +0100
> Alexandre Belloni <alexandre.belloni@...e-electrons.com> wrote:
> 
> > On 04/11/2015 at 07:36:37 -0800, Joshua Clayton wrote :
> > > +	ret = pcf2123_rtc_read_time(dev, &tm);
> > > +	if (ret < 0)
> > > +		return false;
> > > +
> > > +	if (rtc_valid_tm(&tm) < 0) {
> > > +		dev_err(dev, "retrieved date/time is not
> > > valid.\n");
> > > +		return false;
> > > +	}
> > > +
> > 
> > I would remove that test as basically, the date/time will only be
> > valid when OSC_HAS_STOPPED is not set.
> > 
> OSC_HAS_STOPPED really only protects us in case everything else
> looks good, but we've lost power.
> There are other reasons to check for a valid time.
> Specifically, if there is no communication with the device, the spi
> operation will succeed and return all zeros or all ones.
> Since either of these results in an invalid time, it is a nice way
> to probe whether we really have a pcf2123 compatible device.
> 
> I don't think I should remove this test, but I can add a comment

OK but then pcf2123_rtc_read_time actually returns rtc_valid_tm(tm) ;)

The proper course of action is probably to do the OSC_HAS_STOPPED check
in pcf2123_rtc_read_time then you don't even need pcf2123_time_valid().

-- 
Alexandre Belloni, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com
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