linux-kernel - Re: [PATCH] sbs-battery: add option to always register battery

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Message-ID: <20150603141035.GO9880@ci00147.xsens-tech.local>
Date:	Wed, 3 Jun 2015 16:10:35 +0200
From:	Frans Klaver <frans.klaver@...ns.com>
To:	Sebastian Reichel <sre@...nel.org>
CC:	Dmitry Eremin-Solenikov <dbaryshkov@...il.com>,
	David Woodhouse <dwmw2@...radead.org>,
	<linux-pm@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sbs-battery: add option to always register battery

On Wed, Jun 03, 2015 at 03:57:51PM +0200, Sebastian Reichel wrote:
> Hi Frans,
> 
> On Tue, Jun 02, 2015 at 03:14:43PM +0200, Frans Klaver wrote:
> > Commit a22b41a31e53 ("sbs-battery: Probe should try talking to the
> > device") introduced a step in probing the SBS battery, that tries to
> > talk to the device before actually registering it, 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.
> > 
> > Which is of course reasonable. However, it is also very well possible
> > for a device to boot up on wall-power and be connected to a battery
> > later on. The current advice in this situation is to probe the device
> > from userspace if you expect the battery to come on at some point in the
> > future. The downside of this approach is that userspace needs to be
> > aware of the backend of its powersupply, which is inconvenient and going
> > against the point of hardware abstraction.
> > 
> > In some of these cases you do want to register a battery, even if none
> > are attached at the moment. To facilitate this, add a configuration
> > option to try to talk to the device, defaulting to y, thus keeping the
> > current behavior. If unset, the battery will always be registered
> > without checking the sanity of the connection.
> > 
> > Signed-off-by: Frans Klaver <frans.klaver@...ns.com>
> > ---
> > If there's a better place to arrange for this all to happen, or to make this
> > more common across power supplies, I'm perfectly happy to do that work instead.
> > For now this seems like the logical step to take, especially since using device
> > tree was (sensibly) shot down last september [0].
> 
> While I still think, that the HW design is bad,

I'm still interested in learning how we could improve the HW design in
your opinion. Would you say we should be using a non-removable battery?


> I'm basically fine
> with this change based upon your comments. I think it's better to
> make this into a module parameter, though, since that moves the
> decision about this feature from compilation time to module load
> time. This will make it possible to use a generic kernel on your
> device. Maybe something like this could be used:
> 
> module_param(force_load, bool, 0444);
> MODULE_PARM_DESC(force_load,
> 		 "Attempts to load the driver even if the "
> 		 "battery is not connected");

That makes sense. We can work with that.

Thanks,
Frans
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