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

From: Alexandre Belloni <alexandre.belloni@bootlin.com>
To: Miguel Borges de Freitas <miguelborgesdefreitas@gmail.com>
Cc: Rob Herring <robh@kernel.org>, Baruch Siach <baruch@tkos.co.il>,
	a.zummo@towertech.it, Fabio Estevam <festevam@gmail.com>,
	Sascha Hauer <s.hauer@pengutronix.de>,
	Jon Nettleton <jon@solid-run.com>,
	Russell King - ARM Linux admin <linux@armlinux.org.uk>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	devicetree <devicetree@vger.kernel.org>,
	NXP Linux Team <linux-imx@nxp.com>,
	Sascha Hauer <kernel@pengutronix.de>,
	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: Tue, 25 Aug 2020 22:08:51 +0200	[thread overview]
Message-ID: <20200825200851.GI2389103@piout.net> (raw)
In-Reply-To: <CAC4G8N5zUVc0YvT9mCP4BfeQD+KOAo6Rbswo8zqUh_mULa=Xsg@mail.gmail.com>

On 27/07/2020 22:13:42+0100, Miguel Borges de Freitas wrote:
> Russell King - ARM Linux admin <linux@armlinux.org.uk> escreveu no dia
> segunda, 27/07/2020 à(s) 18:30:
> >
> > 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.
> 
> For what it's worth I also tend to agree.
> The patchset, regardless of the property name (that I admit might be
> misleading), is intended at enforcing a mode that the RTC/driver
> should use by default. This mode is strongly related to the hardware
> definition/implementation and its capabilities. While I understand the
> need for the IOCTL interface to solve issues exactly like the
> aforementioned factory example, I fail to see how it can be of any
> help to solve the problem at hand - as it won't likely configure the
> driver to use a different default mode depending on the board. The
> IOCTL interface might also allow the userspace to change this property
> back to the default mode (000) and end up breaking the RTC operation,
> but I guess that's the cost of configurability and, in the end, the
> user's responsibility.
> Any pointers on how to proceed are appreciated.
> 

I would think the simpler way to proceed is to have a device specific
property indicating that standard mode is not available. From the
driver, you can then switch from standard to DSM when this property is
present.

I think it is difficult to come up with a generic property for that as
most other RTCs with level/threshold switching have a fast edge
detection feature that is usually enabled by default. So what they would
require instead is a property indicating that the voltage is ramping
down at a certain rate allowing to disable fast edge detection and
saving a bit of power.

-- 
Alexandre Belloni, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com

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