# One-Key KVM: Mac ↔ Nobara

**Status:** WIP - Scripts created, needs testing  
**Hardware:** Dell U3821DW, MX Keys S, MX Master 3S  
**Updated:** 2026-02-01

## Overview

Software KVM solution to switch the Dell U3821DW ultrawide monitor and Logitech MX peripherals between Mac (MacBook Pro M3) and Nobara Linux using DDC/CI and HID++ commands.

## Hardware Mapping

### Monitor Inputs
| Input | Code | Port | Computer |
|-------|------|------|----------|
| USB-C | 27 | USB-C | Mac |
| HDMI 2 | 18 | HDMI 2 | Nobara |

### Peripheral Channels
| Channel | Code | Connection | Computer |
|---------|------|------------|----------|
| 1 | 0x00 | Bluetooth | Mac |
| 2 | 0x01 | Logi Bolt USB | Nobara |

### Devices
- **Monitor:** Dell U3821DW (38" Ultrawide, DDC/CI enabled)
- **Keyboard:** Logitech MX Keys S (Easy-Switch capable)
- **Mouse:** Logitech MX Master 3S (Easy-Switch capable)
- **Receiver:** Logi Bolt USB (PID: 046d:c548)

## Prerequisites

### Mac
```bash
# Install m1ddc (DDC control for Apple Silicon)
brew install m1ddc

# Install hidapitester (HID control)
curl -sL https://github.com/todbot/hidapitester/releases/latest/download/hidapitester-macos-arm64.zip -o /tmp/hidapitester.zip
unzip -o /tmp/hidapitester.zip -d ~/bin/
chmod +x ~/bin/hidapitester
```

### Nobara (Linux)
```bash
# Install ddcutil
sudo dnf install ddcutil

# Add user to i2c group (requires reboot)
sudo usermod -aG i2c $USER

# Install hidapitester
curl -sL https://github.com/todbot/hidapitester/releases/latest/download/hidapitester-linux-amd64.zip -o /tmp/hidapitester.zip
unzip -o /tmp/hidapitester.zip -d ~/bin/
chmod +x ~/bin/hidapitester

# Udev rule for non-root HID access
echo 'SUBSYSTEM=="usb", ATTRS{idVendor}=="046d", ATTRS{idProduct}=="c548", MODE="0666"' | \
  sudo tee /etc/udev/rules.d/99-logitech-bolt.rules
sudo udevadm control --reload-rules && sudo udevadm trigger
```

## Scripts

### Switch to Nobara (Run on Mac)
**Location:** `~/scripts/to_nobara.sh`

```bash
#\!/bin/bash
# KVM Switch: Mac → Nobara
set -e

echo "Switching to Nobara..."

# Switch Monitor to HDMI 2
m1ddc set input 18

# Switch Mouse to Bolt (Channel 2)
~/bin/hidapitester --vid 046d --pid c548 --usage 0x0001 --usagePage 0xff00 \
    --open --length 7 --send-output "0x10,0x01,0x09,0x1b,0x01,0x00,0x00"

# Switch Keyboard to Bolt (Channel 2)
~/bin/hidapitester --vid 046d --pid c548 --usage 0x0001 --usagePage 0xff00 \
    --open --length 7 --send-output "0x10,0x02,0x09,0x1b,0x01,0x00,0x00"

echo "Done\!"
```

### Switch to Mac (Run on Nobara)
**Location:** `~/scripts/to_mac.sh`

```bash
#\!/bin/bash
# KVM Switch: Nobara → Mac
set -e

echo "Switching to Mac..."

# Switch Monitor to USB-C
ddcutil setvcp 60 27

# Switch Mouse to Bluetooth (Channel 1)
~/bin/hidapitester --vid 046d --pid c548 --usage 0x0001 --usagePage 0xff00 \
    --open --length 7 --send-output "0x10,0x01,0x09,0x1b,0x00,0x00,0x00"

# Switch Keyboard to Bluetooth (Channel 1)
~/bin/hidapitester --vid 046d --pid c548 --usage 0x0001 --usagePage 0xff00 \
    --open --length 7 --send-output "0x10,0x02,0x09,0x1b,0x00,0x00,0x00"

echo "Done\!"
```

## Keyboard Shortcuts

### Mac (Logi Options+)
1. Logi Options+ → MX Keys S → Smart Actions
2. Create "KVM Switch" action
3. Trigger: F12
4. Action: System → Open File → `~/scripts/to_nobara.sh`

### Nobara (GNOME)
1. Settings → Keyboard → Custom Shortcuts
2. Command: `/home/USER/scripts/to_mac.sh`
3. Shortcut: F12

## Troubleshooting

### Monitor doesn't switch
- **DDC/CI disabled:** Monitor Menu → Others → DDC/CI → On
- **USB-C hub blocking DDC:** Connect monitor directly to Mac USB-C port
- **Linux permissions:** Ensure user in `i2c` group, reboot after adding

### Peripherals don't switch
- **Wrong device index:** The 0x01/0x02 values depend on Bolt pairing order
  ```bash
  # Find correct indices on machine with Bolt receiver
  hidapitester --vid 046d --pid c548 --list-detail
  ```
- **Different Bolt PID:** Some receivers have different product IDs
  ```bash
  hidapitester --list | grep 046d
  ```

### HID++ Command Reference
| Byte | Purpose | Values |
|------|---------|--------|
| 0x10 | Report ID (short) | Fixed |
| 0x01/0x02 | Device index | Mouse=0x01, Keyboard=0x02 (may vary) |
| 0x09 | Feature index | Change Host feature |
| 0x1b | Function | Set host |
| 0x00/0x01 | Channel | 0x00=Ch1, 0x01=Ch2, 0x02=Ch3 |

## DDC Input Codes (Dell U3821DW)

| Input | Code | Hex |
|-------|------|-----|
| DisplayPort 1 | 15 | 0x0F |
| DisplayPort 2 | 16 | 0x10 |
| HDMI 1 | 17 | 0x11 |
| HDMI 2 | 18 | 0x12 |
| USB-C | 27 | 0x1B |

## Files Location

Scripts are stored on Mac at `~/scripts/`:
- `to_nobara.sh` - Switch to Nobara (run on Mac)
- `to_mac.sh` - Switch to Mac (copy to Nobara)
- `debug_kvm.sh` - Diagnostic script
- `setup_nobara.sh` - Nobara setup script

## TODO

- [ ] Test monitor switching with DDC/CI enabled
- [ ] Verify Bolt receiver PID on Nobara
- [ ] Confirm device indices for keyboard/mouse
- [ ] Set up keyboard shortcuts on both machines
- [ ] Test bidirectional switching