linux-kernel - Re: [PATCH v3 1/2] dt-bindings: google,cros-ec-keyb: add has-fn-map prop

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Message-ID: <aVzrfwY8SrIAGn-e@google.com>
Date: Tue, 6 Jan 2026 11:01:19 +0000
From: Fabio Baltieri <fabiobaltieri@...omium.org>
To: Krzysztof Kozlowski <krzk@...nel.org>
Cc: Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Benson Leung <bleung@...omium.org>,
	Guenter Roeck <groeck@...omium.org>,
	Tzung-Bi Shih <tzungbi@...nel.org>, Simon Glass <sjg@...omium.org>,
	linux-input@...r.kernel.org, devicetree@...r.kernel.org,
	chrome-platform@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/2] dt-bindings: google,cros-ec-keyb: add has-fn-map
 prop

On Tue, Jan 06, 2026 at 08:27:10AM +0100, Krzysztof Kozlowski wrote:
> On Mon, Jan 05, 2026 at 10:55:51AM +0000, Fabio Baltieri wrote:
> > On Mon, Jan 05, 2026 at 08:52:56AM +0100, Krzysztof Kozlowski wrote:
> > > On Wed, Dec 31, 2025 at 02:35:37PM +0000, Fabio Baltieri wrote:
> > > > Add binding documentation for the has-fn-map property.
> > > > 
> > > > Signed-off-by: Fabio Baltieri <fabiobaltieri@...omium.org>
> > > > ---
> > > >  .../devicetree/bindings/input/google,cros-ec-keyb.yaml    | 8 ++++++++
> > > >  1 file changed, 8 insertions(+)
> > > > 
> > > > diff --git a/Documentation/devicetree/bindings/input/google,cros-ec-keyb.yaml b/Documentation/devicetree/bindings/input/google,cros-ec-keyb.yaml
> > > > index fefaaf46a240..fa24b1cbc788 100644
> > > > --- a/Documentation/devicetree/bindings/input/google,cros-ec-keyb.yaml
> > > > +++ b/Documentation/devicetree/bindings/input/google,cros-ec-keyb.yaml
> > > > @@ -44,6 +44,14 @@ properties:
> > > >        where the lower 16 bits are reserved. This property is specified only
> > > >        when the keyboard has a custom design for the top row keys.
> > > >  
> > > > +  google,has-fn-map:
> > > > +    description: |
> > > > +      The keymap has function key layer. This allows defining an extra set of
> > > > +      codes that are sent if a key is pressed while the KEY_FN is held pressed
> > > > +      as well. The function codes have to be defined in the linux,keymap
> > > > +      property with an offset of keypad,num-rows from the normal ones.
> > > > +    type: boolean
> > > 
> > > You still did not answer to my previous question, why this is not
> > > deducible from the key map (presence of KEY_FN in the map).
> > 
> > The driver behaves differently with the fn layer is present, has to make
> > extra space for the extra codes and enable the logic to use it. I can
> > certainly detect it in runtime, would have to always allocate the extra
> > space even if not needed and check not only that there is an FN key but
> > if there's anything in the second half of the map.
> > 
> > I'm not overly enthusiastic about it, it's a bit wasteful on memory
> > (probably no big deal, half a kb of RAM I guess) and somewhat less
> > defensive to misconfigurations and in general I don't like the new logic
> > to be enabled magically, as a side effect. It'd be extra complexity for
> > the sake of saving one boolean property, but sure if you think that's
> > the way to go then I guess I can implement it that way.
> 
> Driver logic is not an argument here, we don't care about it. You should
> answer why presence of google,has-fn-map in DT makes sense when none of
> the keymaps has KEY_FN. Why this is a valid and desired configuration?
> 

Think I answered that, it felt like a clarer design to me. Anyway I got
the message will send a v4 with runtime detection, it's certainly a
valid option and works out for me.