linux-kernel - Re: [PATCH v2 1/3] dt-bindings: rtc: pcf8523: add DSM pm option for battery switch-over

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Message-ID: <20200727154335.GL1551@shell.armlinux.org.uk>
Date:   Mon, 27 Jul 2020 16:43:35 +0100
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Alexandre Belloni <alexandre.belloni@...tlin.com>
Cc:     Rob Herring <robh@...nel.org>, miguelborgesdefreitas@...il.com,
        a.zummo@...ertech.it, baruch@...s.co.il, shawnguo@...nel.org,
        s.hauer@...gutronix.de, kernel@...gutronix.de, festevam@...il.com,
        linux-imx@....com, devicetree@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] dt-bindings: rtc: pcf8523: add DSM pm option for
 battery switch-over

On Mon, Jul 27, 2020 at 05:41:04PM +0200, Alexandre Belloni wrote:
> On 27/07/2020 16:24:39+0100, Russell King - ARM Linux admin wrote:
> > On Mon, Jul 27, 2020 at 04:49:38PM +0200, Alexandre Belloni wrote:
> > > On 27/07/2020 10:45:53+0100, Russell King - ARM Linux admin wrote:
> > > > > This is but this shouldn't be a DT property as it has to be changed
> > > > > dynamically. I'm working on an ioctl interface to change this
> > > > > configuration.
> > > > 
> > > > Why does it need to be changed dynamically?  If the hardware components
> > > > are not fitted to allow the RTC to be safely used without DSM, then
> > > > why should userspace be able to disable DSM?
> > > 
> > > For RTCs with a standby mode, you want to be able to return to standby
> > > mode.
> > > 
> > > That would happen for example after factory flashing in that common use
> > > case:
> > >  - the board is manufactured
> > >  - Vbackup is installed, the RTC switches to standby mode
> > >  - the board is then booted to flash a system, Vprimary is now present,
> > >    the RTC switches to DSM.
> > > 
> > > At this point, if the board is simply shut down, the RTC will start
> > > draining Vbackup before leaving the factory. Instead, we want to be able
> > > to return to standby mode until the final user switches the product on
> > > for the first time.
> > 
> > I don't think you're understanding what's going on with this proposed
> > patch.  The cubox-i does work today, and the RTC does survive most
> > power-downs. There are situations where it doesn't.
> > 
> > So, let's take your process above.
> > 
> > - the board is manufactured
> > - Vbackup is installed, the RTC switches to standby mode
> > - the board is then booted to flash a system, Vprimary is now present
> > - the board is powered down.  the RTC _might_ switch over to battery
> >   if it notices the power failure in time, or it might not.  A random
> >   sample of units leaving the factory have the RTC in standby mode.
> >   Others are draining the battery.
> > 
> > I'm not saying what you propose isn't a good idea.  I'm questioning
> > why we should expose this in the generic kernel on platforms where
> > it's likely to end up with the RTC being corrupted.
> > 
> 
> Note that I didn't say we should expose settings that are not working
> but it is a different discussion.

It isn't a different discussion - that is exactly what the point of
my emails to you all along have been!

So, can we please have that discussion, it is pertinent to this patch.

-- 
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