From e8e60b6439eed340a611e9d7a5b9bcbd0ef62725 Mon Sep 17 00:00:00 2001 From: Dave Carey Date: Thu, 30 Jul 2026 08:43:36 -0400 Subject: [PATCH] HID: multitouch: Fix stale MT slots when contact count drops to zero MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 Tested-by: Dave Carey Signed-off-by: Benjamin Tissoires --- drivers/hid/hid-multitouch.c | 19 ++++++++----------- 1 file changed, 8 insertions(+), 11 deletions(-) diff --git a/drivers/hid/hid-multitouch.c b/drivers/hid/hid-multitouch.c index 2c41bacab1ca..451c7324e6a0 100644 --- a/drivers/hid/hid-multitouch.c +++ b/drivers/hid/hid-multitouch.c @@ -1321,21 +1321,18 @@ static void mt_touch_report(struct hid_device *hid, * Includes multi-packet support where subsequent * packets are sent with zero contactcount. */ - if (contact_count >= 0) { + if (contact_count > 0) + app->num_expected = contact_count; + else if (app->num_received == 0 && app->prev_scantime != scantime) { /* + * New multi-report frame: + * * For Win8 PTPs the first packet (td->num_received == 0) may * have a contactcount of 0 if there only is a button event. - * We double check that this is not a continuation packet - * of a possible multi-packet frame be checking that the - * timestamp has changed. + * + * Some other devices use a sentinel frame with 0 to release all contacts */ - if ((app->quirks & MT_QUIRK_WIN8_PTP_BUTTONS) && - app->num_received == 0 && - app->prev_scantime != scantime) - app->num_expected = contact_count; - /* A non 0 contact count always indicates a first packet */ - else if (contact_count) - app->num_expected = contact_count; + app->num_expected = 0; } app->prev_scantime = scantime;