[PATCH AUTOSEL 6.18-5.10] HID: multitouch: add MT_QUIRK_STICKY_FINGERS to MT_CLS_VTL

[Sasha Levin <sashal@kernel.org>]

From: DaytonCL <artem749507@gmail.com>

[ Upstream commit ff3f234ff1dcd6d626a989151db067a1b7f0f215 ]

Some VTL-class touchpads (e.g. TOPS0102:00 35CC:0104) intermittently
fail to release a finger contact. A previous slot remains logically
active, accompanied by stale BTN_TOOL_DOUBLETAP state, causing
gestures to stay latched and resulting in stuck two-finger
scrolling and false right-clicks.

Apply MT_QUIRK_STICKY_FINGERS to handle the unreleased contact correctly.

Link: https://gitlab.freedesktop.org/libinput/libinput/-/issues/1225
Suggested-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Tested-by: DaytonCL <artem749507@gmail.com>
Signed-off-by: DaytonCL <artem749507@gmail.com>
Signed-off-by: Benjamin Tissoires <bentiss@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---

LLM Generated explanations, may be completely bogus:

## Analysis of HID Multitouch Quirk Addition

### 1. COMMIT MESSAGE ANALYSIS

The commit clearly describes a **real user-visible bug**:
- VTL-class touchpads (specifically TOPS0102:00 35CC:0104)
  intermittently fail to release finger contacts
- This causes gestures to stay latched - stuck two-finger scrolling and
  false right-clicks
- Links to a libinput bug report:
  https://gitlab.freedesktop.org/libinput/libinput/-/issues/1225
- Has `Tested-by:` from the reporter and `Suggested-by:` from Benjamin
  Tissoires (HID maintainer)
- Signed off by the maintainer (Benjamin Tissoires)

### 2. CODE CHANGE ANALYSIS

The change is **extremely minimal** - adding one line:
```c
MT_QUIRK_STICKY_FINGERS |
```

to the existing `MT_CLS_VTL` class definition. This enables an already-
existing quirk flag (`MT_QUIRK_STICKY_FINGERS`) for VTL-class devices.

The quirk mechanism handles cases where a touchpad firmware fails to
properly report when a finger has been lifted. Without it, the system
thinks the finger is still touching, causing "sticky" gestures. The
quirk infrastructure already exists and is well-tested - this commit
just enables it for another device class.

### 3. CLASSIFICATION

This falls squarely into the **QUIRKS and WORKAROUNDS exception
category**:
- Hardware-specific quirk for buggy/non-compliant devices
- The quirk code already exists in mainline - only enabling it for VTL
  class
- Fixes real-world hardware behavior issues

This is NOT:
- A new feature or API
- A new driver
- Code refactoring

### 4. SCOPE AND RISK ASSESSMENT

| Factor | Assessment |
|--------|------------|
| Lines changed | 1 |
| Files touched | 1 |
| Complexity | Trivially low |
| Subsystem | HID multitouch (mature, stable) |
| Risk | Very low |

The worst case scenario: if the quirk somehow caused issues on a
specific VTL device, it would only affect that device class. But the
current behavior is already broken for affected users, so this is a
clear improvement.

### 5. USER IMPACT

- **Affected users**: Anyone with VTL-class touchpads
- **Symptoms**: Stuck two-finger scrolling, false right-clicks - these
  are highly disruptive daily-use issues
- **Severity**: Medium-high for affected users (makes touchpad
  unreliable)
- **Evidence of real impact**: Linked bug report from libinput tracker

### 6. STABILITY INDICATORS

- `Tested-by:` tag from the reporter who experienced the issue
- Suggested and signed off by HID subsystem maintainer
- The `MT_QUIRK_STICKY_FINGERS` quirk has existed since ~Linux 3.15
- The `MT_CLS_VTL` class has existed since ~Linux 4.13
- Well-established quirk pattern used by many other device classes
  (FLATFROG, LG, ASUS, etc.)

### 7. DEPENDENCY CHECK

- **No dependencies** on other commits
- Both the quirk flag and VTL class exist in all recent stable trees
- Should apply cleanly without modification

### STABLE KERNEL RULES ASSESSMENT

| Criterion | Met? |
|-----------|------|
| Obviously correct and tested | ✅ Yes - trivial change, has Tested-by |
| Fixes a real bug | ✅ Yes - documented hardware issue with bug report |
| Important issue | ✅ Yes - stuck gestures/false clicks affect daily use
|
| Small and contained | ✅ Yes - 1 line in 1 file |
| No new features | ✅ Yes - uses existing quirk mechanism |
| Applies cleanly | ✅ Yes - no dependencies |

### CONCLUSION

This is an **ideal stable backport candidate**:

1. **Minimal risk**: Single-line addition of an existing, well-tested
   quirk flag
2. **Real user benefit**: Fixes stuck gestures and false clicks on VTL
   touchpads
3. **Proper testing**: Has Tested-by tag and maintainer approval
4. **Falls under hardware quirk exception**: Hardware quirks/workarounds
   are explicitly allowed in stable even though they add code
5. **Self-contained**: No dependencies, applies cleanly to stable trees

The fix is small, surgical, obviously correct, and addresses a real
user-visible hardware issue. This is exactly the type of hardware quirk
addition that stable trees should accept.

**YES**

 drivers/hid/hid-multitouch.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/hid/hid-multitouch.c b/drivers/hid/hid-multitouch.c
index 179dc316b4b51..a0c1ad5acb670 100644
--- a/drivers/hid/hid-multitouch.c
+++ b/drivers/hid/hid-multitouch.c
@@ -393,6 +393,7 @@ static const struct mt_class mt_classes[] = {
 	{ .name = MT_CLS_VTL,
 		.quirks = MT_QUIRK_ALWAYS_VALID |
 			MT_QUIRK_CONTACT_CNT_ACCURATE |
+			MT_QUIRK_STICKY_FINGERS |
 			MT_QUIRK_FORCE_GET_FEATURE,
 	},
 	{ .name = MT_CLS_GOOGLE,
-- 
2.51.0