linux-kernel - Handling of USB "Programmable button" controls as KEY

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Message-ID: <6ebbb200-1f2c-450b-8fae-e5e2dd9b6be9@t-8ch.de>
Date:   Tue, 18 May 2021 15:21:40 +0200
From:   Thomas Weißschuh <linux@...ssschuh.net>
To:     linux-input@...r.kernel.org, linux-usb@...r.kernel.org,
        Jiri Kosina <jikos@...nel.org>,
        Benjamin Tissoires <benjamin.tissoires@...hat.com>,
        Hans de Goede <hdegoede@...hat.com>
Cc:     linux-kernel@...r.kernel.org
Subject: Handling of USB "Programmable button" controls as KEY_MACRO# events

Hi everybody,

Would it make sense to map the "Programmable Buttons" control from the
USB HID Consumer page [0] to the linux event codes KEY_MACRO1 ... KEY_MACRO# ?

Those controls are documented in the USB spec as:

"The user defines the function of these
buttons to control software applications or GUI objects."

The KEY_MACRO event codes are documented with:

"Some keyboards have keys which do not have a defined meaning, these keys
are intended to be programmed / bound to macros by the user."

My usecase is the passing of custom keycodes from a programmable keypad
(via QMK[1]) to Linux.
(This would also need new functionality in QMK itself)

Alternatives:

* Send Raw HID from QMK
  * Con: needs a dedicated, nonstandard driver on the host
* Use F-Keys
  * Con: only F13-F19 are usable (F1-F12 are used by normal keyboards, F20-F23
    are repurposed with other keys for X11 compat)

Possible problems:

* There are 65k programmable keys defined by USB but only 30 macro keys are
  supported by Linux.

Thanks,
Thomas

[0] https://www.usb.org/sites/default/files/hut1_22.pdf#section.15.14
[1] https://qmk.fm/