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" > -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
