DDC does not work with docking station that uses DisplayPort/MST on Mac

[qkw24]

Before opening the issue, have you...?

• Searched for existing issues
• Looked through the wiki
• Updated MonitorControl to the latest version (if applicable)

Describe the bug

I have a Dell S2421H connected via HP Thunderbolt Dock G2, the DDC control functionality does not work when using Monitor Control on Mac. However, when using the same dock and monitor on Windows 11, with apps like DisplayBuddy, Monitorian, the DDC control does work, and I did verify the display brightness can be adjusted using DDC. On first glance, it looks like the dock supports DDC, it's either macOS or Monitor Control preventing DDC from working.

Steps to reproduce

1. Connect display to the display port output of the HP Thunderbolt Dock G2.
2. Install Monitor Control on Mac and complete the necessary set up steps.
3. Try to adjust brightness or volume on Dell S2421H monitor using DDC.
4. Dell S2421H monitor does not respond to the adjustments made on Monitor Control.
5. Keeping the same monitor + dock setup, get a Windows 11 laptop and install apps like DisplayBuddy or Monitorian.
6. Observe that the brightness can be adjusted using DDC with Windows 11 + DisplayBuddy/Monitorian.

Expected behavior

Monitor should respond to adjustments.

Anything else?

No response

Environment Information (please complete the following information)

- macOS version: Ventura 13.4.1
- Mac model: MacBook Pro 14-inch, 2021, M1 Max
- MonitorControl version: 4.1.0
- Monitor(s): Dell S2421H
- Apple Silicon/M1 (yes or no): Yes

[Tommmciu]

Same story here with G2 dock, iiyama XUB2792QSU
for DisplayPort DDC is not working, display connected via Thunderbolt->DisplayPort DDC is working. Where can I look for more information? any logs?

[waydabber]

Docks that use DisplayPort MST technology in any form won't properly work with Mac. DDC won't work and the secondary DisplayPort connector will just show the same screen as the primary one. The G2 dock is such unfortunately.

[qkw24]

Just to confirm, even we are not actively using DisplayPort MST (e.g. only having one monitor connected to the dock's DP port), DDC still won't work?

[waydabber]

Yes, this seems to be the case for all other devices as well.

[sidevesh]

@waydabber do we know why things aren't working when a dock is in use ?

[waydabber]

It should work except if the dock uses MST. MacOS generally does not support MST, probably there is some kind of internal addressing/routing issue (?) with i2c communication with hubs that use MST. Did not investigate the details though.

[sidevesh]

@waydabber I tested things MonitorControl with a displaylink dock and ddc does not work with that either,
on Windows things work normally for ddc control with a displaylink dock too.

[waydabber]

Yes, displaylink does not provide DDC API fro macOS. Did try to get in touch with them but it seems like they aren't too eager to get this fixed or provide an API for that.

[sidevesh]

@waydabber is there a thread or somewhere people can post and let displaylink folks know that this is something we need or can we show support in some other way,
and in the meantime keep this open, I think its a critical thing and sad that they aren't keen on implementing this when they have already done that for windows.

[sidevesh]

Also this issue also is about MST docks, which unless we are sure is impossible completely because of Mac OS just can't do this we should keep it open

[waydabber]

DisplayLink is a separate issue. I tried contacting them and offer to work with them on DDC but they just did not follow up, probably understaffed. Lack of MST support is a generic macOS issue. I'll keep this issue open as a "Known issue".