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: <20200825200851.GI2389103@piout.net>
Date:   Tue, 25 Aug 2020 22:08:51 +0200
From:   Alexandre Belloni <alexandre.belloni@...tlin.com>
To:     Miguel Borges de Freitas <miguelborgesdefreitas@...il.com>
Cc:     Russell King - ARM Linux admin <linux@...linux.org.uk>,
        Jon Nettleton <jon@...id-run.com>,
        Rob Herring <robh@...nel.org>, 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 27/07/2020 22:13:42+0100, Miguel Borges de Freitas wrote:
> Russell King - ARM Linux admin <linux@...linux.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