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

From: Russell King - ARM Linux admin <linux@armlinux.org.uk>
To: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Rob Herring <robh@kernel.org>,
	miguelborgesdefreitas@gmail.com, a.zummo@towertech.it,
	baruch@tkos.co.il, shawnguo@kernel.org, s.hauer@pengutronix.de,
	kernel@pengutronix.de, festevam@gmail.com, linux-imx@nxp.com,
	devicetree@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 1/3] dt-bindings: rtc: pcf8523: add DSM pm option for battery switch-over
Date: Mon, 27 Jul 2020 16:24:39 +0100	[thread overview]
Message-ID: <20200727152439.GK1551@shell.armlinux.org.uk> (raw)
In-Reply-To: <20200727144938.GC239143@piout.net>

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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