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

[Krzysztof Kozlowski]

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@xxxxxxxxxxxx>
> > > ---
> > >  .../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?

Best regards,
Krzysztof