linux-kernel - Re: [PATCH] HID: i2c-hid: fix handling of unpopulated devices

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Message-ID: <ZQqemN8P2VKgxhsV@hovoldconsulting.com>
Date:   Wed, 20 Sep 2023 09:26:16 +0200
From:   Johan Hovold <johan@...nel.org>
To:     Doug Anderson <dianders@...omium.org>
Cc:     Johan Hovold <johan+linaro@...nel.org>,
        Jiri Kosina <jikos@...nel.org>,
        Benjamin Tissoires <benjamin.tissoires@...hat.com>,
        linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
        Maxime Ripard <mripard@...nel.org>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        LinusW <linus.walleij@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>
Subject: Re: [PATCH] HID: i2c-hid: fix handling of unpopulated devices

On Tue, Sep 19, 2023 at 11:15:46AM -0700, Doug Anderson wrote:
> On Tue, Sep 19, 2023 at 12:07 AM Johan Hovold <johan@...nel.org> wrote:

> > 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.

Ok, I prefer not hiding away things like that as it obscures what's
really going on, for example, in this case, that you register a device
without really having probed it.

As I alluded to in the commit message, you probably want to be able to
support second-source touchscreen panel followers as well at some point
and then deferring checking whether device is populated until the panel
is powered on is not going to work.

I skimmed the thread were you added this, but I'm not sure I saw any
reason for why powering on the panel follower temporarily during probe
would not work?

> 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...

That was my first idea as well, but conceptually it is more correct to
request resources at probe time and not at some later point when you can
no longer fail probe.

You'd also need to handle the fact that the interrupt may never have
been requested when remove() is called, which adds unnecessary
complexity.

Johan