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>, Baruch Siach <baruch@tkos.co.il>,
	Sascha Hauer <kernel@pengutronix.de>,
	a.zummo@towertech.it, Fabio Estevam <festevam@gmail.com>,
	Sascha Hauer <s.hauer@pengutronix.de>,
	Jon Nettleton <jon@solid-run.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	devicetree <devicetree@vger.kernel.org>,
	NXP Linux Team <linux-imx@nxp.com>,
	Miguel Borges de Freitas <miguelborgesdefreitas@gmail.com>,
	Shawn Guo <shawnguo@kernel.org>,
	linux-arm-kernel <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH v2 1/3] dt-bindings: rtc: pcf8523: add DSM pm option for battery switch-over
Date: Mon, 27 Jul 2020 18:30:51 +0100	[thread overview]
Message-ID: <20200727173051.GM1551@shell.armlinux.org.uk> (raw)
In-Reply-To: <20200727161632.GF239143@piout.net>

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 ...

-- 
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