focaltech_finger_state stores finger coordinates x and y as unsigned
int. When processing relative packets, negative deltas can cause
unsigned integer underflow if the finger moves past the left or bottom
boundary of the touchpad, wrapping the coordinates to values near
UINT_MAX.
When clamping the coordinates in focaltech_report_state(), these
underflowed values are clamped against priv->x_max / priv->y_max instead
of 0, causing the cursor to jump erratically to the opposite edge of the
touchpad.
Change the coordinate variables and limits to signed int so that
negative values resulting from relative movements clamp correctly to 0,
and write the clamped values back to state in focaltech_report_state()
to prevent coordinate wind-up accumulation at the touchpad boundaries.
Fixes: 05be1d079e ("Input: psmouse - support for the FocalTech PS/2 protocol extensions")
Reported-by: sashiko-bot@kernel.org
Link: https://patch.msgid.link/am_tH_F938rK6ask@google.com
Assisted-by: Antigravity:gemini-3.6-flash
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
7f9c8c6716