linux-kernel - Re: [PATCH v3 00/19] Report power supply from hid-logitech-hidpp

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Date:   Mon, 05 Jun 2017 18:22:13 +0200
From:   Bastien Nocera <hadess@...ess.net>
To:     Dave Hansen <dave.hansen@...el.com>, Jiri Kosina <jikos@...nel.org>
Cc:     Benjamin Tissoires <benjamin.tissoires@...hat.com>,
        linux-input@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 00/19] Report power supply from hid-logitech-hidpp

On Mon, 2017-06-05 at 07:53 -0700, Dave Hansen wrote:
> On 06/05/2017 06:09 AM, Bastien Nocera wrote:
> > > I agree with Dave. If there is no solution found in time for
> > > -rc5, 
> > > reverting to previous state would be the proper way to go.
> > 
> > I don't see how it's possible to retroactively fix user-space.
> 
> It's not possible to retroactively change userspace.  That why the
> kernel tries so hard not to break it in the first place.  Although
> this
> is in "minor annoyance" territory for me at the moment, this patch
> causes a clear, user-visible issue with new kernels.
> 
> The right way to do this is to have the kernel export the data in a
> way
> that does not confuse old userspace.  Perhaps we should separate out
> "power supplies that run the system" from "power supplies in a
> perihperal".

There's already such a property for it, "scope". I think that you don't
realise that it's this version of UPower you're using (one major API
version behind the current one) is buggy when it comes to handling
kernel-created "power_supply".

It's just that UPower used to do this itself, in user-space, and that
it gets thoroughly confused when it accesses both the battery from
user-space and kernel-space.

> And, no, a config option isn't the right thing either.

Because...? It's the best way to avoid exposing the feature for ancient
user-spaces. The battery information will be gathered from user-space.
It doesn't regress either.