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

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Message-ID: <61dcf8c7-2dcb-4173-fbbd-9adf3412edb7@redhat.com>
Date:   Tue, 18 May 2021 15:44:52 +0200
From:   Hans de Goede <hdegoede@...hat.com>
To:     Thomas Weißschuh <linux@...ssschuh.net>,
        linux-input@...r.kernel.org, linux-usb@...r.kernel.org,
        Jiri Kosina <jikos@...nel.org>,
        Benjamin Tissoires <benjamin.tissoires@...hat.com>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: Handling of USB "Programmable button" controls as KEY_MACRO#
 events

Hi,

On 5/18/21 3:21 PM, Thomas Weißschuh wrote:
> 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)

I think the idea is good, but AFAICT the HUT does not actually assign
any usage codes in the consumer-page for this. It simply points to the
Button usage-page, which means things conflict with e.g. mouse and joystick
buttons and I do not see any dedicated codes in the table
"Table 15.1: Consumer Page" so I'm not sure how to interpret the spec. here ...

I guess there is something which we can do with the report's application here,
since the code dealing with HID_UP_BUTTON is already doing a switch-case
on field->application to differentiate between mouse and gaming buttons.

I guess interpreting an application of HID_CP_CONSUMER_CONTROL in combination
with using the buttons usage-page as wat the HUT is trying to specify and
thus map that the first 30 codes in that combination to KEY_MACRO1 - 30
might make sense.

Regards,

Hans

> 
> 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/
>