[PATCH 0/3] Input: add ABS_TWIST and support it in HID and Wacom

[Siarhei Vishniakou <svv@google.com>]

Some styli can report rotation along their long ("barrel") axis,
commonly referred to as "twist". While the HID specification defines
this usage as HID_DG_TWIST (0x000d0041) in the Digitizers page, there is
currently no corresponding evdev axis code to expose it to userspace.

Because evdev has no MSC_SCAN equivalent for axes, drivers and OEMs
currently work around this by mapping stylus twist onto unrelated axes
such as ABS_Z or ABS_RZ. As a result, userspace input stacks must
maintain device-specific workarounds to know which axis carries twist.

This series introduces a dedicated ABS_TWIST (0x1d) axis for single-tool
devices such as styli and wires it up in the HID subsystem:

1. Input: Define ABS_TWIST (0x1d) in input-event-codes.h and document it
   in event-codes.rst with resolution in units/radian.
2. HID: Map HID_DG_TWIST (0x000d0041) to ABS_TWIST in hid-input, compute
   its resolution in units/radian, and add KUnit test coverage.
3. HID: wacom: Report pen twist on ABS_TWIST in addition to ABS_Z so
   that userspace can use the standard axis while maintaining backward
   compatibility with existing applications that expect ABS_Z.

Siarhei Vishniakou (3):
  Input: add ABS_TWIST event code
  HID: input: map Digitizers Twist usage to ABS_TWIST
  HID: wacom: report ABS_TWIST alongside ABS_Z for pen twist

 Documentation/input/event-codes.rst    |  6 ++
 drivers/hid/hid-debug.c                |  1 +
 drivers/hid/hid-input-test.c           | 87 ++++++++++++++++++++++++++
 drivers/hid/hid-input.c                |  5 ++
 drivers/hid/wacom_wac.c                | 35 +++++++++--
 include/uapi/linux/input-event-codes.h |  1 +
 6 files changed, 129 insertions(+), 6 deletions(-)

-- 
2.56.0.455.g6c5a503b38-goog