Date:	Wed, 24 Sep 2014 15:38:49 +0100
From:	Mark Rutland <mark.rutland@....com>
To:	Frans Klaver <frans.klaver@...ns.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Dmitry Eremin-Solenikov <dbaryshkov@...il.com>,
	David Woodhouse <dwmw2@...radead.org>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <Pawel.Moll@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Randy Dunlap <rdunlap@...radead.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>
Subject: Re: [RFC PATCH 1/2] sbs-battery: add forced instantiation from
 device tree

On Wed, Sep 24, 2014 at 03:22:22PM +0100, Frans Klaver wrote:
> On Wed, Sep 24, 2014 at 02:16:55PM +0100, Mark Rutland wrote:
> > On Wed, Sep 24, 2014 at 02:11:17PM +0100, Frans Klaver wrote:
> > > In some cases you want to instantiate a battery even before it is
> > > attached; it is perfectly reasonable for a device to start up on
> > > wall-power and be connected to a battery later. The current advice is to
> > > instantiate a device explicitly in the kernel, or probe for the device
> > > from userspace. The downside of these approaches is that the user needs
> > > to keep the information related to the i2c battery in different places,
> > > which is inconvenient.
> > 
> > This really sounds like a Linux policy issue rather than something that
> > should be described in dt.
> > 
> > Presumably there's a reason we sanity cehck this in the first place.
> > What happens while the battery isn't plugged in? What can fail, and how?
> 
> It was introduced in a22b41a31e53 "sbs-battery: Probe should try talking
> to the device", saying:
> 
>   "this driver doesn't actually try talking to the device at probe time,
>   so if it's incorrectly configured in the device tree or platform data
>   (or if the battery has been removed from the system), then probe will
>   succeed and every access will sit there and time out. The end result
>   is a possibly laggy system that thinks it has a battery but can never
>   read status, which isn't very useful."
> 
> That's a reasonable thing to do, but it breaks just the feature I need.
> Besides that, the driver provides us with a gpio that indicates battery
> presence, which will also be useless if the device isn't present at
> probe time. That commit also doesn't take into account the fact that a
> battery could be removed after probing without any problems, leaving the
> system in the same state as before the probe change.
> 
> Now if the battery isn't plugged in, it is never detected after it has
> been attached, unless you take action from userspace. Basically you
> don't know your battery level until it has been explicitly probed.
> 
> We might also reduce the severity of the sanity check failure to produce
> a warning instead of an error. This would achieve that a developer might
> be warned that the battery isn't present, but also allow my use case
> where the battery may not be present at boot time. Was that what you
> meant with policy by the way?

In general, properties in general shouldn't tell the kernel what to do.
They should tell the kernel details of the hardware that it can then use
to make informed decisions. In this case, the suggested property is
purely a software detail, as the hardware isn't any different in
situations you would or would not want the property.

You mention that there's a GPIO that can be used to detect the battery
presence. Why can't the driver always probe and then on check for the
presence of the battery dynamically using that GPIO? That should cover
both cases.

Mark.
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