External monitor: HiDPI only via USB-C?

[ribbon]

I have been reading about the issues with external monitors in MacOS. I have a 4K monitor: when I plug in directly to my MBA M1 USB-C port, everything works great, and I can select scaled HiDPI modes other than the default 1920x1080. However, when I use DisplayPort via the dock I have (Dell D6000) that doesn't work and I need to resort to BetterDummy if I want to select anything other 1920x1080 that doesn't look awful.

Is there any kind of limitation or reason why DP>DP doesn't support HiDPI, but USBC>USBC works without a hiccup?

Thank you.

 

[joevt]

The D6000 is a USB-C dock, not a Thunderbolt dock. Since it supports USB 3.x, it only has two lanes of DisplayPort for a DisplayPort Alt Mode connection. It's probably limited to DisplayPort 1.2 which has a max link rate of HBR2 which is 8.64 Gbps total for two lanes which gives a limit of 4K 30Hz.

Actually, the manual says the HDMI port is the one that uses DisplayPort Alt Mode. A display connected using DisplayPort Alt Mode is directly connected to the GPU so it will have best performance then.

The DisplayPort ports of the D6000 use DisplayLink. DisplayLink uses video over USB 3.0 which is only 4 Gbps but it uses compression to get 4K 60Hz (if you connected two 4K60 displays then each would be limited to 2 Gbps). I don't know how good the compression is. 4K60 is normally 16 Gbps (or 12.8 Gbps for 8bpc) but DisplayLink is doing it using 2 Gbps. The visually lossless compression that DisplayPort 1.4 has (called Display Stream Compression or DSC) can do 4K60 using 6.4 Gbps (or 4.3 Gbps if you use DSC@8bpp).

Anyway, with DisplayLink displays, macOS does not offer HiDPI scaling modes that are greater than the display size. This is a limit of the DisplayLink driver. Ideally, you would want the DisplayLink driver to get a 5K framebuffer from macOS and have the GPU downscale it to 4K for the DisplayLink compression over USB. BetterDummy is a workaround to provide that functionality. You should send feedback to DisplayLink that you want their driver to provide this functionality.

A Thunderbolt dock supports two 17.28 Gbps (four lanes of HBR2 each) DisplayPort connections or one 25.92 Gbps (four lanes of HBR3) and one 8.64 Gbps (four lanes of HBR) connection. So you can connect two 4K 60 displays and they will behave as if they were directly connected to the GPU so they will have much better performance than DisplayLink displays.

 

[ribbon]

Thank you so much for the detailed response joevt. The reason why I keep using my D6000 is because I think DisplayLink is the only workaround to have more than one screen plugged into my M1 MBA. Am I wrong? Would a Thunderbolt dock achieve the same result (with much better performance as you described?).

Thank you.

 

[joevt]

The original M1 Macs can only connect one non-DisplayLink display total to their Thunderbolt ports. A Thunderbolt dock won't help adding additional displays. But what you can do is connect a DisplayLink dongle for additional displays to a Thunderbolt dock. Then a display connected to the Thunderbolt dock directly will have full GPU performance up to 4K120Hz or 5K60Hz or 6K60Hz. Additional displays can be connected to the DisplayLink dongle. Your D6000 can act as the DisplayLink dongle connected to the Thunderbolt dock - all of its functionality should be usable as if it was connected directly to the M1 Mac (but remember, with the original M1 Macs, that you can't use the DisplayPort Alt Mode functionality of the D6000 while having a display connected directly to the Thunderbolt dock).