Re: [RFC PATCH 1/2] sbs-battery: add forced instantiation from device tree

[Mark Rutland <mark.rutland@arm.com>]

On Wed, Sep 24, 2014 at 04:14:48PM +0100, Frans Klaver wrote:
> On Wed, Sep 24, 2014 at 03:38:49PM +0100, Mark Rutland wrote:
> > 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.
> 
> Ok, that makes sense.
> 
> > 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.
> 
> I would say that this was the case before [1] was done. The GPIO is
> optional and if not configured, the presence or absence of the battery
> is detected by checking a status register much like probe() currently
> does. It seems all cases were covered before that patch. If you worry
> about speed, you should use the GPIO. I wonder if we might be able to
> revert [1] without doing much harm.

But reverting that would re-introduce the lag on some systems, no? Given
the wording of the original commit I would guess that the GPIO wasn't
available. Perhaps Olof or Anton can enlighten us?

In the cases where a GPIO is available, I think we should be able to be
less pessimistic. Is a GPIO available in your case?

Mark.

> 
> Thanks,
> Frans
> 
> [1] a22b41a31e53 "sbs-battery: Probe should try talking to the device"
>