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From:   Doug Anderson <dianders@chromium.org>
Date:   Tue, 19 Sep 2023 11:15:46 -0700
Message-ID: <CAD=FV=XBG7auVVyHn5uvahSZZxp5qBfp4+A9NwFqahdN6XrbZA@mail.gmail.com>
Subject: Re: [PATCH] HID: i2c-hid: fix handling of unpopulated devices
To:     Johan Hovold <johan@kernel.org>
Cc:     Johan Hovold <johan+linaro@kernel.org>,
        Jiri Kosina <jikos@kernel.org>,
        Benjamin Tissoires <benjamin.tissoires@redhat.com>,
        linux-input@vger.kernel.org, linux-kernel@vger.kernel.org,
        Maxime Ripard <mripard@kernel.org>,
        Dmitry Torokhov <dmitry.torokhov@gmail.com>,
        LinusW <linus.walleij@linaro.org>,
        Rob Herring <robh+dt@kernel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
        Conor Dooley <conor+dt@kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@vger.kernel.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@vger.kernel.org>
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Hi,

On Tue, Sep 19, 2023 at 12:07=E2=80=AFAM Johan Hovold <johan@kernel.org> wr=
ote:
>
> > c) To solve this in the core, we have to make sure we properly handle
> > (without hanging/failing) multiple partially-conflicting devices and
> > devices that might acquire resources in arbitrary orders.
> >
> > Though the above solution detecting the pinctrl conflicts sounds
> > appealing and I'm currently working on prototyping it, I'm still not
> > 100% convinced. I'm worried about the above downsides.
>
> Yes, I agree that we'd need to take a broader look at this and not just
> focus on the immediate pinctrl issue.

OK. FWIW, I got blocked on trying to solve this in the core
automatically by just using the conflicting "pinctrl" entries. There
are probably some ways to get it solved, but none of them are easy.


> > Personally, I feel like we could add information to the device tree
> > that would help us out. The question is: is this an abuse of device
> > tree for something that Linux ought to be able to figure out on its
> > own, or is it OK? To make it concrete, I was thinking about something
> > like this:
> >
> > / {
> >   tp_ex_group: trackpad-exclusion-group {
> >     members =3D <&tp1>, <&tp2>, <&tp3>;
> >   };
> > };
> >
> > &i2c_bus {
> >   tp1: trackpad@10 {
> >     ...
> >     mutual-exclusion-group =3D <&tp_ex_group>;
> >   };
> >   tp2: trackpad@20 {
> >     ...
> >     mutual-exclusion-group =3D <&tp_ex_group>;
> >   };
> >   tp3: trackpad@30 {
> >     ...
> >     mutual-exclusion-group =3D <&tp_ex_group>;
> >   };
> > };
> >
> > Then the device core would know not to probe devices in the same
> > "mutual-exclusion-group" at the same time.
> >
> > If DT folks are OK with the "mutual-exclusion-group" idea then I'll
> > probably backburner my attempt to make this work on the pinctrl level
> > and go with that.
>
> I expressed something along these lines in the thread above:

I'm going to try coding up the above to see how it looks. Assuming
nothing comes up, I'll try to have something in the next few days.


>         It seems we'd need some way to describe the devices as mutually
>         exclusive...
>
> but given that we had prior art for handling simple cases and due to
> lack of time, I left it on the ever-growing todo list.
>
> But regardless of what a long-term proper solution to this may look
> like, we need to fix the regression in 6.6-rc1 by restoring the old
> behaviour.

OK, fair enough. I'll take a look at your patch, though I think the
person that really needs to approve it is Benjamin...

Style-wise, I will say that Benjamin really wanted to keep the "panel
follower" code out of the main probe routine. Some of my initial
patches adding "panel follower" looked more like the results after
your patch but Benjamin really wasn't happy until there were no
special cases for panel-followers in the main probe routine. This is
why the code is structured as it is.

Thinking that way, is there any reason you can't just move the
i2c_hid_init_irq() into __do_i2c_hid_core_initial_power_up()? You
could replace the call to enable_irq() with it and then remove the
`IRQF_NO_AUTOEN` flag? I think that would also solve the issue if you
wanted to use a 2nd source + the panel follower concept? Both devices
would probe, but only one of them would actually grab the interrupt
and only one of them would actually create real HID devices. We might
need to do some work to keep from trying again at every poweron of the
panel, but it would probably be workable? I think this would also be a
smaller change...

-Doug