Home

SL4A TouchScreen — Linux Kernel Driver

Linux kernel driver for the Surface Laptop 4 (AMD) touchscreen, providing standard HID single-touch, pen, and raw multi-touch via a V0 HID-over-SPI transport on the AMD FCH SPI controller.

Device: MSHW0231 (045E:0C19), 72×48 mutual-capacitance grid
Transport: HID-over-SPI V0 protocol (AMD Cezanne FCH SPI controller V2)
Kernel: 7.1.3-2-cachyos (DKMS sl4a-touch/1.1.0)
Alignment: ~85% with Windows TouchPenProcessor0C19.dll pipeline

Architecture

┌──────────────────────────────────────────────────────────────┐
│  Userspace                                                     │
│  libinput / X11 / Wayland ← evdev ← hid-multitouch            │
├──────────────────────────────────────────────────────────────┤
│  Kernel                                                        │
│                                                                 │
│  ┌─────────────┐   ┌──────────────┐   ┌───────────────────┐  │
│  │ spi-hid-core │   │  spi-hid-core │   │    spi-amd         │  │
│  │  HID LL      │ ← │  Heatmap + MT │ ← │  AMD FCH SPI V2   │  │
│  │  transport   │   │  Pipeline     │   │  PIO controller    │  │
│  └──────┬───────┘   └──────────────┘   └────────┬──────────┘  │
│         │                                         │             │
├─────────┼─────────────────────────────────────────┼─────────────┤
│ Hardware│                                         │             │
│         │            SPI bus (12 MHz)             │             │
│  MSHW0231 Touch MCU ◄──────────────────────────────             │
│  045E:0C19                                                      │
└─────────────────────────────────────────────────────────────────┘

Feature Status

Feature	Status
HID report descriptor (936 bytes, 100% wire-read)	Complete
Single-touch + pen (Report ID 0x40 and 0x01)	Stable
Multi-touch CCL pipeline (peak gate + flood-fill + Hungarian)	Functional
Touch ellipse (per-blob eigenvalues)	Working
Stationary lock (eliminates pinch-to-zoom jitter)	Working
Cold boot auto-retry handshake	Working
2-finger tracking	Excellent
3-finger tracking	Good
4-finger tracking	Partial

Navigation

• Protocol: HID over SPI V0 — Wire format, DESCREQ, SET_FEATURE activation
• Touch Pipeline — Peak gate, CCL, centroid, Hungarian, EMA, emission
• Config Table — Values extracted from TouchPenProcessor0C19.dll
• Hardware Reference — MSHW0231, AMD FCH SPI controller, GPIO
• Build & Install — DKMS, module parameters, troubleshooting
• Reverse Engineering — Methodology, decompilation, known gaps

Quick Install

git clone git@github.com:Syax89/SL4A_TouchScreen.git
cd SL4A_TouchScreen
sudo ./tools/install.sh
# → Reboot, driver auto-loads via udev + /etc/modprobe.d/spi-hid.conf