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From:   Stephen Boyd <swboyd@chromium.org>
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Date:   Mon, 2 May 2022 19:18:13 -0700
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Subject: Re: [PATCH v2 2/2] Input: cros-ec-keyb - skip keyboard registration
 for switches compatible
To:     Doug Anderson <dianders@chromium.org>
Cc:     Dmitry Torokhov <dmitry.torokhov@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>, patches@lists.linux.dev,
        chrome-platform@lists.linux.dev,
        Krzysztof Kozlowski <krzk@kernel.org>,
        Rob Herring <robh+dt@kernel.org>, devicetree@vger.kernel.org,
        Benson Leung <bleung@chromium.org>,
        Guenter Roeck <groeck@chromium.org>,
        Hsin-Yi Wang <hsinyi@chromium.org>,
        "Joseph S. Barrera III" <joebar@chromium.org>
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Quoting Doug Anderson (2022-05-02 18:06:39)
> Hi,
>
> On Mon, May 2, 2022 at 3:02 PM Stephen Boyd <swboyd@chromium.org> wrote:
> >
> > Quoting Doug Anderson (2022-05-02 10:02:54)
> > > On Fri, Apr 29, 2022 at 4:31 PM Stephen Boyd <swboyd@chromium.org> wrote:
> > > >
> > > >
> > > > diff --git a/drivers/input/keyboard/cros_ec_keyb.c b/drivers/input/keyboard/cros_ec_keyb.c
> > > > index eef909e52e23..1bbe2987bf52 100644
> > > > --- a/drivers/input/keyboard/cros_ec_keyb.c
> > > > +++ b/drivers/input/keyboard/cros_ec_keyb.c
> > > > @@ -536,6 +536,12 @@ static int cros_ec_keyb_register_matrix(struct cros_ec_keyb *ckdev)
> > > >         u32 *physmap;
> > > >         u32 key_pos;
> > > >         unsigned int row, col, scancode, n_physmap;
> > > > +       bool register_keyboard;
> > > > +
> > > > +       /* Skip matrix registration if no keyboard */
> > > > +       register_keyboard = device_get_match_data(dev);
> > > > +       if (!register_keyboard)
> > > > +               return 0;
> > > >
> > > >         /*
> > > >          * No rows and columns? There isn't a matrix but maybe there are
> > >
> > > As per my comments in patch #1, I wonder if it makes sense to delete
> > > the "No rows and columns?" logic and settle on the compatible as the
> > > one true way to specify this.
> > >
> >
> > Ok. My only concern is that means we have to check for both compatibles
> > which is not really how DT compatible strings work. The compatible
> > string usually finds the more specific compatible that is first in the
> > list of compatibles in DT. You're essentially proposing that the
> > switches compatible could be first or last, the order doesn't matter.
>
> It's not quite what I was proposing. I think my summary really sums it up:

Alright, I'm glad I misunderstood.

>
> 1. If you have a matrix keyboard and maybe also some buttons/switches
> then use the compatible: google,cros-ec-keyb
>
> 2. If you only have buttons/switches but you want to be compatible
> with the old driver in Linux that looked for the compatible
> "google,cros-ec-keyb" and required the matrix properties, use the
> compatible: "google,cros-ec-keyb-switches", "google,cros-ec-keyb"
>
> 3. If you have only buttons/switches and don't need compatibility with
> old Linux drivers, use the compatible: "google,cros-ec-keyb-switches"
>
> ...but just to say it another way:
>
> * If you have the compatible "google,cros-ec-keyb-switches" I mean to
> say that you _only_ have switches and buttons. You'd _never_ have this
> compatible string if you truly have a matrix keyboard. If you have
> this, it will always be first.
>
> * If you only have switches and buttons but you care about backward
> compatibility then you can add a fallback compatible second:
> "google,cros-ec-keyb"
>
> * In order for the fallback compatible to be at all useful as a
> fallback (it's only useful at all if you're on an old driver), if you
> specify it you should pretend that you have matrix properties even
> though you don't really have them, just like we used to do.
>

Another important point is that the matrix properties are willfully
ignored by the new driver if the "google,cros-ec-keyb-switches"
compatible is present. Maybe it should be "google,cros-ec-no-keyb" to
describe the true intent, i.e. ignore the keyboard properties. Or
"google,cros-ec-keyboardless". I think it's confusing that I put
"switches" in the compatible. It really should be about not registering
the keyboard input device.

Anyway, I agree that we don't need to use the matrix keyboard properties
to figure out what to do. In fact, it isn't possible to remove the
properties if "google,cros-ec-keyb" is present, so checking for them is
redundant.