HID: multitouch: Fix stale MT slots when contact count drops to zero

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The INGENIC 17EF:6161 touchscreen (Lenovo Yoga Book 9 14IAH10) reports
HID_DG_CONTACTCOUNT=0 in the frame immediately following the last finger
lift rather than omitting the frame entirely.  In mt_touch_report() the
existing code only updates num_expected when contact_count is non-zero,
so a zero contact count on the first packet of a new frame leaves
num_expected at its previous value (e.g. 2 for a two-finger gesture).
The sync check "num_received >= num_expected" then evaluates "0 >= 2"
and never fires, preventing INPUT_MT_DROP_UNUSED from releasing the
stale slots.  Those slots remain active in the kernel MT layer until the
next touch, at which point they are released in a batch alongside the
new contact — causing the userspace event consumer to miss the
intervening finger-up sequence and corrupt its gesture session state.

Fix by resetting num_expected to 0 when contact_count is zero and
num_received is still 0 (i.e., this is the first and only packet of the
frame, not a continuation packet in a multi-packet sequence).  With
num_expected=0 the sync check "0 >= 0" fires immediately, calling
input_mt_sync_frame() which drops the stale slots via
INPUT_MT_DROP_UNUSED.

The num_received==0 guard is critical: continuation packets in a
multi-packet frame arrive after at least one contact has already been
processed (num_received>0), so they are correctly excluded from this
path and the existing multi-packet logic is unaffected.

Signed-off-by: Dave Carey <carvsdriver@gmail.com>
Tested-by: Dave Carey <carvsdriver@gmail.com>
Signed-off-by: Benjamin Tissoires <bentiss@kernel.org>

...

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Dave Carey 2026-07-30 08:43:36 -04:00 • committed by Benjamin Tissoires

parent 1c942462c3

commit e8e60b6439

1 changed files with 8 additions and 11 deletions

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19

drivers/hid/hid-multitouch.c

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