Add comprehensive touchpad troubleshooting documentation

- Document ITE8353 touchpad issue on little-rascal
- List all attempted solutions and current status
- Provide next steps for further investigation
- Include useful debugging commands and references

documentation/TOUCHPAD_TROUBLESHOOTING.md

@@ -0,0 +1,98 @@
+# Touchpad Troubleshooting - Little Rascal (Lenovo Yoga Slim 7)
+
+## Problem
+The touchpad on little-rascal (Lenovo Yoga Slim 7 14ARE05) is not working. The trackpad is detected as an I2C HID device but is being misidentified as a sensor hub instead of a proper touchpad.
+
+## Hardware Details
+- **Model**: Lenovo Yoga Slim 7 14ARE05
+- **Touchpad Controller**: ITE8353 (I2C HID)
+- **Device Path**: `/sys/bus/i2c/devices/i2c-ITE8353:00`
+- **HID Device**: `0018:048D:8353.0001`
+- **ACPI ID**: `PNP0C50` (HID-over-I2C precision touchpad)
+
+## Investigation Results
+
+### Device Detection
+The touchpad controller is properly detected by the kernel:
+- I2C device exists at `i2c-ITE8353:00`
+- Driver binding: `i2c_hid_acpi`
+- HID device: `0018:048D:8353.0001`
+
+### Problem: Misidentified as Sensor Hub
+The device is being bound to `hid-sensor-hub` driver instead of a proper touchpad driver like `hid-multitouch`.
+
+### Attempted Solutions
+
+1. **Added I2C HID kernel modules**:
+   - `i2c_hid`, `i2c_hid_acpi`, `hid_multitouch`
+   - Added to both `boot.kernelModules` and `boot.initrd.availableKernelModules`
+
+2. **Added kernel parameters**:
+   - `i2c_hid.debug=1`
+   - `acpi_enforce_resources=lax`
+   - `i2c_hid_acpi.probe_defer=1`
+
+3. **Added udev rules**:
+   - Attempted to unbind from `hid-sensor-hub` and bind to `hid-multitouch`
+   - Set proper permissions for the device
+
+4. **Enabled libinput**:
+   - Configured touchpad settings in `modules/desktop/input.nix`
+   - Added input utilities (`evtest`, `xinput`, `libinput-gestures`)
+
+### Current Status
+- ❌ Touchpad still not functional
+- ✅ Device is detected by kernel
+- ❌ Device is misidentified as sensor hub
+- ❌ No input events generated
+
+## Next Steps to Try
+
+### Option 1: BIOS Settings
+Check BIOS for touchpad mode settings:
+- Look for "Touchpad Mode" setting
+- Try switching between "Basic" and "Advanced" modes
+- Some laptops have I2C/PS2 mode selection
+
+### Option 2: Kernel Patch
+This appears to be a known issue requiring a kernel patch. Research:
+- Linux kernel patches for ITE8353 touchpad support
+- Check if newer kernels have better support
+- Look for device-specific quirks in the kernel
+
+### Option 3: Force PS/2 Mode
+If I2C mode cannot be made to work:
+- Try to enable PS/2 emulation mode in BIOS
+- Some laptops can fall back to PS/2 touchpad mode
+
+### Option 4: Custom Driver
+- Check if ITE provides Linux drivers
+- Look for community-developed drivers for this specific controller
+
+### Option 5: Firmware Update
+- Update laptop firmware/BIOS
+- Some touchpad issues are resolved with firmware updates
+
+## Useful Commands for Further Debugging
+
+```bash
+# Check device binding
+ls -la /sys/bus/i2c/devices/i2c-ITE8353:00/driver
+
+# Check HID devices
+cat /sys/bus/i2c/devices/i2c-ITE8353:00/0018:048D:8353.0001/modalias
+
+# Monitor udev events
+udevadm monitor --environment --udev
+
+# Test input events
+evtest
+
+# Check kernel modules
+lsmod | grep -E "(i2c|hid)"
+```
+
+## References
+- [NixOS Hardware Configuration](https://github.com/NixOS/nixos-hardware)
+- [Linux I2C HID Documentation](https://www.kernel.org/doc/html/latest/input/devices/i2c-hid.html)
+- [Libinput Documentation](https://wayland.freedesktop.org/libinput/doc/latest/)