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: <20200727173051.GM1551@shell.armlinux.org.uk>
Date:   Mon, 27 Jul 2020 18:30:51 +0100
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Alexandre Belloni <alexandre.belloni@...tlin.com>
Cc:     Jon Nettleton <jon@...id-run.com>, Rob Herring <robh@...nel.org>,
        Miguel Borges de Freitas <miguelborgesdefreitas@...il.com>,
        a.zummo@...ertech.it, Baruch Siach <baruch@...s.co.il>,
        Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Sascha Hauer <kernel@...gutronix.de>,
        Fabio Estevam <festevam@...il.com>,
        NXP Linux Team <linux-imx@....com>,
        devicetree <devicetree@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <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 06:16:32PM +0200, Alexandre Belloni wrote:
> On 27/07/2020 17:55:50+0200, Jon Nettleton wrote:
> > > So, can we please have that discussion, it is pertinent to this patch.
> > >
> > 
> > Thinking about this some more, I believe whether or not an IOCTL
> > interface is in the works or needed is irrelevant.  This patch
> > describes the hardware and how it is designed and the topic of
> > discussion is if we need a simple boolean state, or if we need
> > something that could be used to support dynamic configuration in the
> > future.  I would rather make this decision now rather than keep
> > tacking on boolean config options, or revisit a bunch of device-tree
> > changes.
> 
> Something that would describe the hardware is a property telling whether
> the filter is present on VDD, not a property selecting DSM. The driver
> can then chose to avoid standard mode when needed.

Whether DSM needs to be enabled or not is _not_ just a function of
whether there is a filter present or not.

The requirement in the data sheet is that when the VDD supply drops
below 2.5V, it does not fall more than 0.7V/ms.  That can be
achieved many different ways, not only by adding a resistive filter
to the VDD supply to the RTC.  It could also be achieved via the design
of the power supply - for example, having a large enough reservoir
capacitor to ensure under all loads that the VDD supply will not fall
faster than 0.7V/ms.

There are many ways to meet this requirement.

Adding a DT property to indicate whether the filter is present or not
is definitely not the right approach.  Should we also add properties
for every possible solution to this problem.

	vdd-has-filter;
	psu-has-large-capacitors;
	... etc ...

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