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: <20200727152439.GK1551@shell.armlinux.org.uk>
Date:   Mon, 27 Jul 2020 16:24:39 +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 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.

Now, I question your idea that units should leave the factory without
the RTC being programmed.  We know that lovely systemd goes utterly
bonkers if the system time is beyond INT_MAX.  If the RTC leaves
standby mode containing a date which we translate beyond INT_MAX,
systemd will refuse to boot the system, and the user will have no
way to set the correct time.  The user returns the device to the
supplier as faulty...

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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