### System details

HP laptop (ELAN0709 I2C touchpad), Omarchy 4.0

### What's wrong?

# ELAN I2C Touchpad Not Working on Older Laptops — Analysis & Fix

## Problem

Touchpad completely non-functional on an older HP laptop running Omarchy (Arch Linux + Hyprland). No cursor movement, no tap-to-click, no scrolling.

## Environment

- **Laptop:** Older HP laptop (ELAN0709 I2C touchpad)
- **OS:** Omarchy (Arch Linux, kernel 7.1.9-arch1)
- **Compositor:** Hyprland 0.55+ (Lua config)
- **Touchpad:** ELAN0709:00 04F3:31BF (I2C HID)
- **BIOS:** Already on latest version

---

## Investigation

### 1. Initial Config Review

The `input.lua` had several issues that were **not the root cause** but needed fixing:

- Unicode smart quotes (`¨flat¨`) in `accel_profile` line caused a Lua parse error
- `tap` was used instead of `tap_to_click` (renamed in Hyprland 0.55+)
- `accel_profile` was misplaced inside `touchpad` block (it's a top-level `input` option)

### 2. Kernel Driver Status

```
hid_multitouch    36864  0
i2c_hid_acpi      12288  0
i2c_hid           45056  1 i2c_hid_acpi
```

All modules loaded. Touchpad detected in `/proc/bus/input/devices` and `/dev/input/event*`.

### 3. Raw Event Test

Reading `/dev/input/event*` for 5 seconds returned **0 events**. The kernel driver registered the device but zero events were reaching userspace.

### 4. dmesg — The Smoking Gun

```
i2c_hid_acpi i2c-ELAN0709:00: i2c_hid_get_input: incomplete report (14/3583)
```

The `i2c_hid_acpi` driver was receiving corrupted/incomplete HID reports from the ELAN touchpad (14 bytes instead of expected 3583).

### 5. Blacklist Attempt (Failed)

```bash
echo "blacklist i2c_hid_acpi" | sudo tee /etc/modprobe.d/touchpad-fix.conf
```

After reboot: **touchpad completely vanished** from `/proc/bus/input/devices`. The `i2c_hid_acpi` driver is the only one that binds to this ACPI device — blacklisting it left the touchpad undetected.

### 6. Resolution

```bash
sudo rm /etc/modprobe.d/touchpad-fix.conf
sudo reboot
```

After removing the blacklist and rebooting, the touchpad returned and worked. The `incomplete report` warning in dmesg was **harmless** — a known ELAN quirk where the IRQ line stays high after data transfer, causing a spurious report. The driver handles it gracefully.

---

## Root Cause

The `i2c_hid_acpi` driver's `incomplete report (14/3583)` error is a **known kernel bug** with ELAN I2C touchpads. It's caused by:

1. ELAN touchpad firmware takes ~7-8ms to pull IRQ back to high after data transfer
2. The I2C host reads incomplete data before the touchpad is ready
3. This generates a kernel error message but **does not prevent the touchpad from working**

The touchpad was not actually broken — the driver was working. The initial investigation revealed that zero raw events were reaching the test script, but after removing the interfering blacklist and rebooting, the driver resumed normal operation.

---

## Fix Applied

| Change | Purpose |
|--------|---------|
| Added `tap_to_click = true` to `touchpad` block | Enable tap-to-click (correct option name for Hyprland 0.55+) |
| Fixed Unicode quotes in `accel_profile` line | Resolve Lua parse error preventing config from loading |
| Removed `/etc/modprobe.d/touchpad-fix.conf` | Undo blacklist that broke touchpad detection entirely |

---

## Lessons Learned

1. **Don't blacklist `i2c_hid_acpi`** on ELAN touchpads — it's the only driver that binds to the ACPI device
2. The `incomplete report` warning is cosmetic on most ELAN devices
3. Hyprland 0.55 Lua config renamed `tap` → `tap_to_click` and `accel_profile` stays under `input`, not `input.touchpad`
4. Always test raw events (`/dev/input/event*`) to distinguish hardware vs software issues
5. Check `dmesg` before touching config — the kernel log told us the real story

---

## Related Kernel Bug

This is tracked in multiple places:

- [Bugzilla #215747](https://bugzilla.kernel.org/show_bug.cgi?id=215747) — Regression: Mouse gets sluggish after suspend/resume
- [Patch by Kai-Heng Feng](https://patchwork.kernel.org/project/linux-input/patch/[email protected]/) — Adds `I2C_HID_QUIRK_BOGUS_IRQ` for ELAN devices