DDC (Hardware) Brightness Control via DP Port of ThunderBolt Dock for M1 Mac

[DanielYang59]

Hi there,

I'm not sure if this is a already-covered topic but I searched the "closed issue" and didn't find any match.

The problem is: I currently connect my M1 MacBook Pro 13 inch to a Dell WD22TB4 ThunderBolt dock, and could I get DDC (hardware) brightness control to work for my DELL P2217H monitor through DP or HDMI port?

Here are several experiments I did:

Hardware brightness does work in:

• Connect the monitor directly to the TB3 port of laptop through a DP to USB-C cable.
• Connect the monitor to any TB4 port of the dock through the same DP to USB-C cable.

Doesn't seem to work if:

• Connect the monitor to the USB-C DP port of the dock through the same DP to USB-C cable.
• Connect the monitor to either DP port of the dock through DP to DP cable.

As I don't understand the mechanism, I want to know if this is expected behaviour (DDC works only when connect directly to TB port)?

Also the reason I want to get hardware brightness to work is to free up a TB4 port so I could connect more other device.

Thanks for your time.

Regards,
Haoyu

Some additional info:
MacOS 13.2.1 (latest as of writing)
WD22TB4 on 04 July 2022 firmware (latest as of this writing)

[waydabber]

Some docks relay DDC communication properly and some don't - it's a hit or miss. The app is sending the DDC but there might be an issue with the dock preventing it reaching the display. If it works directly connected, then it is firmly a dock issue. The app does not have any built in limitation/bias against docks and dongles, a properly designed one should relay DDC I2C communication to the display properly. You can check with the manufacturer for maybe a firmware upgrade or their stance on this.

Note that the M1 13" can only support a single external display so if the dock has support for multiple displays it might have extra stuff inside and it might indicate that the video signal is not directly sent to the screen but instead there is some translation going on which probably affects DDC if badly implemented.

[DanielYang59]

Hi @waydabber,

Thanks a ton for your explaination.

Sorry I might not make the model of the dock clear. It's the ThunderBolt 4 dock WD22TB4 from Dell and from my knowledge is at least not a DisplayLink dock. Yes I could confirm I'm aware of the external display limitation for 13 inch M1 Macs and all those tests were done with one display attached only.

I would try to raise an issue to the Dell support and see if I could get anything from them.

Thanks for your time.

[artgoldberg]

Hi @DanielYang59 and @waydabber

I have two questions, both related to my Likely problem with VGA to USB C cable discussion.

1. Which display port (VGA) to USB C cable do you use with BetterDisplay @DanielYang59? I ask, because I'd like to buy one. See my discussion for more details.
2. I'm surprised by your "the M1 13" can only support a single external display" comment, as the M1 14" supports multiple external displays, as do other smaller Intel MacBooks. But perhaps the M1 13" has special limitations.

Thanks guys!
Arthur

[waydabber]

Hi @artgoldberg - yes, the M1 13" has only the entry level M1 chip while the 14" MBP has M1 Pro/Max with multiple external display support.

Most cheap USB-C to DisplayPort cables will do as the Mac provides native DisplayPort signal via USB-C.

[DanielYang59]

Hi @waydabber thanks for your reply. However I'm afraid DisplayPort to USB-C cables are not what @artgoldberg has been looking for. Shall I understand him correctly, he should be looking for a VGA (D-sub) to USB-C adapter. And unfortunately I cannot give any advice here as I completely have no experience with such adapters.

[waydabber]

Hmm. Meaning connecting a VGA out (from an old PC for example) to an USB-C display? That's an interesting setup. There might be a chain of converters that could do that.

[DanielYang59]

I assume it's very unlikely. As he mentioned in the Discussion, he @artgoldberg has been working with a ViewSonic VA2256-mhd monitor, which comes with a VGA, a HDMI and a DP port. I would suggest switching to a DP (display side) to USB-C (laptop side) cable unless there's a good reason not to.

[waydabber]

Yes. Meanwhile I found the other post of @artgoldberg #1914 and I understood the issue better + tried to answer (could not provide too much actual help I am afraid). :)

[DanielYang59]

Hi @artgoldberg,

Thanks for bringing up the topic.

1. Unfortunately, I was working with monitors without VGA ports and the cable I mentioned earlier was a DisplayPort to USB-C cable. Also worth noting is that such cable (DP to USB-C) does NOT have chip builtin, meaning it's likely just a "physical" switch of the connector shape, as USB-C cable could carry DP signal, more details here.

2. The shortage for native support of multiple external displays of M1 chip have been quite intensively explored, for example this blog, as Apple opted for dropping of Multi-Stream Transfer (MST) support in their baseline chips (M1 or M2). Meanwhile as you mentioned, M1 Pro chip would natively support up to two external displays while M1 Max supports up to three.

Also I used "native" here meaning there's still a workaround if you decide to hold on to your MacBooks with M1/M2 chips. You could opt for adapters with DisplayLink chip which should enable you to hook more than one displays in "Extend" mode. You could find a list of such adapters/docks here.

Sorry for unable to help with the VGA adapter issue.

Regards,
Daniel

[DanielYang59]

Hi @waydabber sorry I would need to bring up this discussion again. I'm currently working with a MacBook Pro 14-inch with M1 Pro chip, connecting to a Philips 24M1N5500Z monitor through DP cable from a Dell WD19TB dock (lastest firmware for dock). Interestingly I'm getting the same behaviour as my previous Dell WD22TB4 dock where the hardware brightness of the monitor cannot by adjusted. Also the display could look dimmer or brighter on the surface but the brightness on the OSD remain unchanged, so I assume BetterDisplay might have applied a mask (hue) or something but the backlight brightness might remain unchanged. Is there any workaround in this case? Would connecting the display through HDMI cable possibly resolve the issue (if it's possible I would try a HDMI cable as I currently don't have any at hand)?

Also I assume DELL is a pretty recognized in this sector and WD22TB4/WD19TB are their most advanced docks (though not designed specifically for Macs). I don't quite understand how things could go so wrong.

Thanks again for your time.

Regards,
Daniel

[waydabber]

To me it seems like the dock uses MST then when multiple displays connected. Honestly I am not sure how this affects DDC with this dock. The best course of action seems to be to connect the display directly or simply use a compatible dock if possible.

[artgoldberg]

Hi @DanielYang59 and @waydabber

If there are any helpful tests I can do with a MacBook 14" with an M1 Pro chip and two external monitors please let me know. E.g., I don't mind buying some DP cables, testing them, and then returning them.

But I might be slow to respond.

Regards
ARthur

[DanielYang59]

Hi @waydabber just to follow up. I tried to swap only the MacBook M1 Pro with a Windows 10 laptop (Huawei Matebook X Pro 2019 with 8th Gen I5 CPU), and keep everything else (display, dock, dock DP to display DP connection) untouched, DDC control seems to work just fine on this setup with the Twinkle Tray APP.

Hope this information would help pin the problem.

Regards,
Daniel

[waydabber]

Windows generally handles MST hubs. MacOS basically has zero support for these so I am not sure what could be done to improve the situation. :(

[DanielYang59]

That's bad news @waydabber . I might try to contact Dell support and see if I could raise the issue to them. But chances are pretty slim as it's related to non-Dell laptops.