Agent Skills: MacBook Desktop Mode

MacBook Desktop Mode

A holistic configuration guide for running a MacBook as an always-on-AC desktop workstation. Solves two interconnected problems: USB devices (especially USB 1.1 audio) disappearing during sleep/wake cycles, and unnecessary battery degradation from constant charge cycling.

When to Use This Skill

• USB microphone or audio device disappears after sleep/wake
• Battery is cycling unnecessarily on a plugged-in Mac
• Setting up a MacBook as a permanent desktop workstation
• Configuring pmset for always-on-AC use
• Setting up a powered USB hub with uhubctl for software-controlled USB resets
• Enhancing the AudioDeviceMonitor Swift daemon with self-healing recovery

Root Cause Diagnosis Framework

Before applying fixes, diagnose the specific failure mode. This framework was developed from empirical analysis of a MacBook Pro M3 Max with an Antlion USB Microphone (VID 0x2F96, PID 0x0200).

DarkWake Cycling

macOS performs frequent DarkWake (partial maintenance wake) cycles — typically every 15 minutes overnight. During DarkWake:

• CPU wakes for Power Nap, network keepalive, Siri
• USB bus is partially powered but devices aren't fully re-enumerated
• USB 1.1 Full Speed devices lack Link Power Management (LPM) and can't negotiate graceful resume
• After multiple cycles, the XHCI controller drops the device from the IO registry

Diagnostic command — check sleep/wake history:

pmset -g log | grep -E "Sleep |Wake |DarkWake" | tail -30

USB 1.1 Device Limitations

USB 1.1 devices (12 Mbps, USBSpeed = 1) are the most fragile across sleep/wake:

• No Link Power Management protocol
• No USB selective suspend negotiation
• Often lack serial numbers (iSerialNumber = 0), making re-identification after bus reset unreliable

Diagnostic command — check device properties:

ioreg -r -c IOUSBHostDevice -l | grep -A 20 "DEVICE_NAME"

Key fields: USBSpeed (1=Full, 2=High, 3=Super), iSerialNumber (0 = no serial), IOPowerManagement.DriverPowerState.

USB Handle Contention

Applications like Chrome hold direct USB user client handles (AppleUSBHostDeviceUserClient) for WebRTC/WebAudio. These stale handles prevent clean re-initialization after sleep.

Diagnostic command — check who has USB handles:

ioreg -r -c IOUSBHostDevice -l | grep -B5 "IOUserClientCreator"

Battery Micro-Cycling

On an always-AC Mac with no charge limit, the battery cycles between the ML-predicted level and actual charge. DarkWake cycles consume power, causing repeated charge/discharge micro-cycles.

Diagnostic command — check daily charge range:

ioreg -r -c AppleSmartBattery -l | grep -E "DailyMinSoc|DailyMaxSoc|Temperature|CycleCount"

• Temperature is in centidegrees (divide by 100 for Celsius)
• DailyMinSoc/DailyMaxSoc show today's charge swing range

Phase 1: Power Configuration for Desktop Mode

1.1 Set Charge Limit to 80%

System Settings → Battery → Charging Optimization → "Limit to 80%"

On Apple Silicon, once at the limit the Mac enters AC bypass mode — power flows directly from charger to system board. The battery sits electrically disconnected, eliminating both calendar aging and cycle aging.

1.2 Set sleep=0 on AC

For an always-on-AC desktop, system sleep creates more problems than it solves:

# Disable system sleep on AC only (battery settings unchanged)
sudo pmset -c sleep 0

# Verify
pmset -g custom | grep -A1 "AC Power" | grep sleep

What this preserves:

• Display sleep still works (displaysleep=10 on AC)
• Battery sleep settings unchanged (sleep=1 for travel)
• ttyskeepawake=1 still honored

What this eliminates:

• DarkWake cycling (root cause of USB dropout)
• Battery micro-cycling during maintenance wakes
• Thermal cycling (repeated cold↔warm transitions)

Cost: ~5-8W idle draw on Apple Silicon. No fan spin. ~$5-8/year in electricity.

1.3 Verify Power Configuration

# Full power settings
pmset -g custom

# Battery health snapshot
ioreg -r -c AppleSmartBattery -l | grep -E "Temperature|CycleCount|DailyMinSoc|DailyMaxSoc|MaxCapacity|IsCharging"

# Active power assertions
pmset -g assertions

Phase 2: Hardware Layer — Powered USB Hub

2.1 Why a Powered Hub

A powered USB hub creates a USB session boundary. The Mac's XHCI controller maintains a session with the hub (a robust USB 2.0+ device with serial number). The hub independently maintains sessions with connected devices. Sleep/wake only stresses the Mac↔Hub link.

Additionally, the hub enables software-controlled USB port reset via uhubctl — the equivalent of a physical unplug/replug without touching hardware.

2.2 Hub Selection Criteria

Requirements:

• Externally powered (not bus-powered) — must have its own power supply
• uhubctl-compatible chipset — supports per-port power switching
• USB 2.0+ hub with serial number for reliable re-identification

Compatible chipsets (from the uhubctl project):

• VIA VL805/VL812/VL817
• Realtek RTS5411
• Genesys Logic GL850G/GL3510

2.3 uhubctl Setup

# Install
brew install uhubctl

# List compatible hubs (must have powered hub connected)
uhubctl

# Cycle all ports (2-second off period)
uhubctl -a cycle -d 2

# Cycle specific port on specific hub
uhubctl -a cycle -p 1 -l 1-1 -d 2

Phase 3: Software Layer — AudioDeviceMonitor Enhancement

The reference implementation is a Swift daemon (AudioDeviceMonitorRunner.swift) deployed as a macOS launchd KeepAlive agent. It combines:

1. Priority enforcement (original) — sets default input/output to highest-priority available device
2. Device guardian (v2) — state machine tracking, disappearance detection, recovery cascade
3. Wake detection — IOKit power notifications via IORegisterForSystemPower
4. Heartbeat — 60-second periodic check for silent drops
5. Recovery cascade — uhubctl port cycle → retry → Telegram notification

3.1 State Machine

                    ┌──────────┐
          ┌────────▶│ PRESENT  │◀──────────────┐
          │         └────┬─────┘               │
          │              │ device gone          │
          │              ▼                      │
          │     ┌──────────────┐               │
          │     │ DISAPPEARED  │               │
          │     │  (3s debounce)│               │
          │     └──────┬───────┘               │
          │            │ still missing          │
          │            ▼                        │
          │     ┌──────────────┐     found     │
          │     │  RECOVERING  │───────────────┘
          │     │ uhubctl cycle│
          │     └──────┬───────┘
          │            │ max attempts
          │            ▼
          │     ┌──────────────┐
          │     │     DEAD     │
          └─────│  (Telegram)  │
          replug└──────────────┘

3.2 Key Architecture Decisions

• IOKit power callbacks instead of NSWorkspace — no AppKit dependency, works in headless launchd daemons
• IOKit message constants defined manually — Swift can't import iokit_common_msg() C macros; values computed from sys_iokit (0xe0000000) | message_code
• Synchronous uhubctl/curl calls — acceptable because the main RunLoop queues CoreAudio callbacks during blocking; no events lost, just delayed by recovery time
• Telegram via curl subprocess — no library dependency; credentials loaded from a dotenv file at startup
• Single-threaded via main queue — all CoreAudio callbacks, heartbeat, and power notifications dispatch to .main; no locks needed

3.3 Build and Deploy

The daemon is a single-file Swift program compiled into a self-contained binary:

# Compile
swiftc -O -framework CoreAudio -framework IOKit \
    -o /path/to/output/audio-device-monitor \
    AudioDeviceMonitorRunner.swift

# Codesign for launchd (ad-hoc, no Apple Developer account needed)
codesign -s - -f -i com.yourorg.audio-device-monitor /path/to/output/audio-device-monitor

Deploy as a launchd agent (user-level, KeepAlive):

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
    <key>Label</key>
    <string>com.yourorg.audio-device-monitor</string>
    <key>ProgramArguments</key>
    <array>
        <string>/path/to/output/audio-device-monitor</string>
    </array>
    <key>KeepAlive</key>
    <true/>
    <key>RunAtLoad</key>
    <true/>
    <key>ProcessType</key>
    <string>Background</string>
    <key>EnvironmentVariables</key>
    <dict>
        <key>HOME</key>
        <string>/Users/yourusername</string>
    </dict>
    <key>StandardOutPath</key>
    <string>/path/to/logs/audio-device-monitor-stdout.log</string>
    <key>StandardErrorPath</key>
    <string>/path/to/logs/audio-device-monitor-stderr.log</string>
</dict>
</plist>

Load: launchctl load ~/Library/LaunchAgents/com.yourorg.audio-device-monitor.plist

3.4 Configuration (in Swift source)

Priority lists and guarded devices are defined as constants at the top of the Swift source:

1. inputPriorities / outputPriorities — ordered arrays, highest priority first
2. guardedDevices — array of GuardedDevice(name:, isInput:) for disappearance monitoring
3. Tuning constants: heartbeatInterval (60s), disappearDebounce (3s), maxRecoveryAttempts (3)
4. Notification credentials: loaded from a dotenv file (path configured in loadCredentials())

After changes, recompile and restart the launchd agent.

Phase 4: Verification and Monitoring

4.1 Battery Health Monitoring

# Quick battery snapshot
ioreg -r -c AppleSmartBattery -l | grep -E "Temperature|CycleCount|DailyMinSoc|DailyMaxSoc|MaxCapacity"

# Full power state
pmset -g batt
system_profiler SPPowerDataType | grep -E "Cycle|Condition|Maximum|Charging"

With the 80% charge limit set and sleep=0 on AC:

• DailyMinSoc and DailyMaxSoc should converge (no swing)
• Temperature should stay under 3500 (35°C) at steady state
• CycleCount accumulation should slow dramatically

4.2 USB Device Health

# Current USB device tree
system_profiler SPUSBDataType

# IOKit details for a specific device
ioreg -r -c IOUSBHostDevice -l | grep -A 25 "DEVICE_NAME"

# Kernel USB assertions (shows which devices prevent sleep)
pmset -g assertions | grep USB

4.3 Audio Monitor Logs

# Tail live logs (adjust path to your log location)
tail -50 /path/to/logs/audio-device-monitor-stderr.log

# Key log patterns to watch for:
# "Starting v2 — device guardian mode"     → v2 features active
# "System woke — checking devices"         → wake detection working
# "ALERT: '<device>' disappeared"          → device dropout detected
# "recovered via uhubctl!"                 → automated recovery succeeded
# "recovery FAILED"                        → manual replug needed

Anti-Patterns

| Anti-Pattern | Why It Fails | Correct Approach | | ---------------------------------------- | ------------------------------------------------------------ | -------------------------------------------------- | | Changing powernap=0 to reduce DarkWake | Trades USB stability for missed iCloud sync, Find My, etc. | Set sleep=0 on AC — eliminates DarkWake entirely | | Disabling Optimized Battery Charging | Allows battery to charge to 100% — worse for longevity | Set explicit 80% charge limit | | Killing Chrome to release USB handles | Whack-a-mole; any WebRTC app can grab handles | Powered hub creates session boundary | | Polling system_profiler for USB status | Expensive subprocess, 2-3 second latency | CoreAudio property listeners are instant | | Using NSWorkspace.didWakeNotification | Requires AppKit/NSApplication — won't work in launchd daemon | IORegisterForSystemPower callback | | Resetting USB via IOKit when device gone | Can't reset a device that's already dropped from IO registry | uhubctl power-cycles the physical port |

See Also

• kokoro-tts:realtime-audio-architecture — Complementary skill covering audio playback patterns (PortAudio, GIL contention, jitter elimination, device hot-switching). This skill handles the system/USB layer; that one handles the application/playback layer.