linux-kernel - Re: [PATCH v2 2/2] dt-bindings: google,cros-ec-keyb: add use-fn-overlay prop

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Message-ID: <aVO6syiW5uUtOXRi@google.com>
Date: Tue, 30 Dec 2025 11:42:43 +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 v2 2/2] dt-bindings: google,cros-ec-keyb: add
 use-fn-overlay prop

On Tue, Dec 30, 2025 at 08:26:56AM +0100, Krzysztof Kozlowski wrote:
> On 29/12/2025 16:39, Fabio Baltieri wrote:
> > Sure so I guess this falls into the "describe the hardware feature"
> > category, so is the suggestion to rename it to something like
> > "has-fn-key"? That would be the hardware feature.
> 
> 
> Maybe, but then I would follow up with - what about "alt", "ctrl",
> "shift" and "fn" keys? And what about combinations alt+ctrl, alt+shift?
> And also caps-lock? And why exactly this has to be even specified if
> matrix map already has the FN key?

Fn works as a key layer, that is when the key is pressed the rest of the
matrix uses a different keymap, different codes. The driver has to make
extra space in the internal data structures for reading the new codes
from the keymap property and use them when they are specified. The other
combinations you mentioned behave normally, nothing special about them.

Looking at the input driver history the other driver that [used to] have
this was using a platform data field called use_fn_map, guess I'll go
with use-fn-map then (that one has never been ported over to dt).