Re: "proximity" attribute for input devices

[Sebastian Reichel <sebastian.reichel@collabora.com>]

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Hi,

On Fri, Mar 03, 2023 at 03:05:41PM -0800, Jason Gerecke wrote:
> Apologies for the delay in replying.
> 
> First off: as an immediate action, I'm thinking of changing the Wacom
> driver to do lazy creation of the power_supply device. This would mean
> that rather than creating it at the same time as the input device, we
> would wait until we receive some kind of affirmative indication of a
> battery being present. Most Wacom peripherals report battery status in
> a "heartbeat" report that is sent every few seconds, so there wouldn't
> be much change for the typical user (just a few-second delay between
> connecting the hardware and a power_supply device being created). In
> the case of the "component" devices that are built into laptops and
> other computers, however, the battery status is only reported while
> the pen is actually in proximity. For users like you who don't own (or
> never use) a pen, this means that our driver would never create a
> power_supply device -- preventing GNOME from showing a battery that
> isn't relevant. I'm working on a patch set that does this but need a
> bit more time to finish it.
>
> That's only a partial solution, however. If a component user were to
> bring a pen into prox even just briefly, then a power_supply device
> would be created and not go away until the user reboots. The pen's
> battery status will become stale at some point in time though --
> self-discharge, use of the pen on another device, and even just simple
> irrelevance if the user hasn't used the pen in a while (do I care that
> the pen which I left at home is at 50% battery?). I agree that it
> makes sense for userspace to hide the battery after a while due to
> things like this. Are there new signals that the kernel should be
> providing to userspace (e.g. an attribute that indicates if we're in
> communication with power_supply? an attribute signaling that data
> might be stale)? Or are the signals that are already provided
> sufficient (e.g. should GNOME just hide power_supply devices that are
> in an "Unknown" state? or maybe hide power_supplies which haven't been
> updated in more than 1 hour?)

Can't you just unregister the power-supply device if there hasn't
been any heartbeat for some minutes and go back to the init state
(i.e. the one where you are waiting for a heartbeat to appear for
the first time)?

> I've added the power_supply list and maintainer for some additional
> viewpoints on the second paragraph... If there are questions about how
> the Wacom hardware and its battery reporting works, I'm happy to
> provide answers...

I think the problem you have is quite specific to the Wacom
hardware. Usually wireless devices are either connected or
disconnected and drivers unregister the battery device on
disconnect. So I think this logic belongs into Wacom specific
code and not in generic power-supply core code.

-- Sebastian

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