DisplayFixer

A small macOS menu-bar app that keeps an external display at 10-bit 4:4:4 when it's driven through a Synaptics VMM7100 USB-C to HDMI adapter. Without it, these adapters tend to come back as washed-out 8-bit YCbCr 4:2:2 every time the Mac wakes from sleep.

I built it for my own setup: an M4 Pro on macOS 26.4.1 running a 4K 165 Hz display through a Cable Matters VMM7100 adapter. That's the only place it's been tested, so your mileage may vary.

The problem

The adapter shows up to macOS as a DisplayPort sink (DP 1.4 / HBR3) and converts to HDMI 2.1 on the far side. At 4K 165 Hz, 10-bit 4:4:4 only fits down the DisplayPort link with DSC (Display Stream Compression) turned on. After a wake, macOS often fails to bring DSC back up, so it falls back to the densest format that fits without it: 8-bit YCbCr 4:2:2. It also stops offering 10-bit 4:4:4 at all until the link is renegotiated. The manual fix most people land on is to unplug and replug the cable, then toggle HDR on/off or reselect 10-bit. DisplayFixer does that part for you.

What it does

On every wake, and when a display reconnects, it:

1. Resets the VMM7100 over USB, which renegotiates the link into a DSC-capable state. This is the same "reset board" command that Synaptics' own Windows tool (VMMHIDTool) sends.
2. Toggles HDR on and back off, which leaves the wire at 10-bit 4:4:4 in SDR.

How it works

A few details that weren't obvious and are worth writing down:

• The reset is three USB HID SET_REPORT control transfers to the Synaptics chip (06CB:7100). On macOS 26 you can't open the USB interface anymore (it returns kIOReturnExclusiveAccess), which is probably what broke BetterDisplay's version of this reset. DisplayFixer sends the packets through a device-level DeviceRequest instead, and that still works.
• It reads the live link state straight from the DCP (IOAVVideoInterfaceGetLinkData) and corroborates that with a scan of the connection colour elements for the live-DSC bit.
• The 10-bit 4:4:4 comes from the HDR toggle (SLSDisplaySetHDRModeEnabled). Turning HDR on forces 10-bit, and therefore DSC; turning it back off leaves you at 10-bit 4:4:4 without HDR.

Requirements

• An Apple Silicon Mac (it uses the DCP display path).
• macOS 13 or later. Developed on macOS 26.4.1.
• A VMM7100-based USB-C to HDMI adapter (Cable Matters and several others use this chip). With a different chip the app simply finds nothing to reset and does nothing.

Build and install

./build.sh                          # builds DisplayFixer.app (ad-hoc signed)
mkdir -p ~/Applications
cp -R DisplayFixer.app ~/Applications/
open ~/Applications/DisplayFixer.app

Then open the menu-bar item and turn on "Start at login" so it survives reboots.

Usage

The menu-bar glyph shows the state: ✓ healthy, ▲ degraded, ⊝ no external display.

• Fix now: run the whole sequence by hand. The screen blanks for a second while the adapter resets.
• Fix automatically on wake: on by default.
• Reset on every wake: off by default, so it only resets when it detects the bad state. Turn it on if you'd rather reset every wake regardless; the brief screen blank is the tradeoff.
• Start at login, Open log, Quit.

Logs go to ~/Library/Logs/DisplayFixer/displayfixer.log.

Caveats

• It uses private macOS frameworks (SkyLight and the IOKit USB internals). Apple can change those in any release, and it's the reason the app can't be sandboxed or shipped on the App Store.
• It only handles the first external display, which is fine for a single-monitor setup.
• The reset packets were captured from one adapter's firmware, so they might not match every VMM7100 revision. If they don't, the reset does nothing rather than breaking anything.
• Build it yourself from source with build.sh. A prebuilt unsigned binary gets blocked by Gatekeeper.

How it was found

The research/ folder has the throwaway probes I used to work the mechanism out: reading the wire color format off the DCP, poking at the IOAVService link controls, the HDR toggle, and the USB reset. They're kept around in case anyone wants to see how the pieces fit together.

Credits

• djrobx/USBResetter reverse-engineered the VMM7100 reset command from Synaptics' Windows VMMHIDTool.
• waydabber/vmm7100reset (MIT) is the Swift port that shipped in BetterDisplay; the reset byte sequence used here comes from it.
• The Apple Silicon DDC/DCP groundwork comes from m1ddc and MonitorControl.

See CREDITS.md.

License

MIT. See LICENSE.