Do you have an Intel Mac that you can use AGDCDiagnose on to get the OUI and FW version for the chip? Or perhaps a DPCD dump from Linux will be sufficient.
I have a Acer XV273K display. It has DisplayPort 1.4 and HDMI 2.0b inputs.
I’m using macOS Big Sur 11.5.1 on a Mac mini 2018 which has an HDMI 2.0b port connected to Intel iGPU (Intel UHD Graphics 630) and Thunderbolt ports also connected to iGPU. I also have a W5700 eGPU (6 DisplayPort 1.4 outputs supporting DSC, one of them is a USB-C port with DisplayPort Alt Mode).
The CAC-1080 doesn’t give me an HDR option but the Cable Matters 201388, Apple USB-C Digital AV Multiport Adapter, and the Mac mini 2018’s built-in HDMI port can all give me HDR from any of the ports (Intel or AMD). The Cable Matters and Apple adapter require 30Hz from Thunderbolt. 60Hz works from the other ports including the Mac mini’s HDMI port…
For the Apple adapter 30Hz is expected from Thunderbolt since it’s two lanes DisplayPort 1.2 and DisplayPort 1.2 doesn’t support 4:2:0.
The Mac mini’s HDMI 2.0b port is using four lanes HBR2 with 4:4:4 10bpc input and 4:2:0 10bpc output 4K 60Hz (according to DPCD registers obtained by using AGDCDiagnose command – the HDMI adapter MegaChips MCDP2920A4 does 4:4:4 to 4:2:0 conversion).
For the Cable Matters 201388 adapter, from Thunderbolt:
4K 30Hz HDR (no format conversion info listed in AGDCDIagnose)
For Intel GPUs, AGDCDiagnose doesn’t give input or output pixel format information.
Since DisplayPort 1.2 doesn’t support 4:2:0, and there’s no 4:4:4 to 4:2:0 conversion, then 30Hz max for 10bpc is expected. It seems to me that the Synaptics VMM6100 should be able to do format conversion, (DPCD register 5 indicates so) but it’s not happening. The MegaChips has register 80 specifying HDMI for DWN_STRM_PORTX_CAP, but the VMM6100 says DisplayPort. I guess I should find the Format conversion info in the DisplayPort 1.2 spec to see how that’s supposed to work (DisplayPort 1.4 spec cannot be found online). Maybe Apple has special software for the Megachips (which it uses in many products including the Apple TV and M1 Mac mini).
For the Cable Matters 201388 adapter, from AMD W5700:
4K 60Hz 10bpc HDR, HBR2 x4, 4:2:0
In this case, DWN_STRM_PORTX_CAP is HDMI so I don’t know what happened in the Thunderbolt case. No Format conversion is required with this connection since the AMD supports DisplayPort 1.4 which supports 4:2:0 which is required for 10 bpc which is required for HDR.
For CAC-1085 from AMD W5700:
4K 60Hz 4:4:4 8bpc (no HDR option).
Maybe macOS is dumb or too smart – it won’t do HDR unless some specific conditions are met. The Cable Matters adapter meets those conditions. So what’s the difference?
One thing strange about the Cable Matters is that it’s showing DSC Support: 0, which seems wrong for a HDMI 2.1 adapter. Maybe it’s reporting the DSC capability of the display. My display is only HDMI 2.0 so DSC is not required. DSC would be required if the adapter was only getting two lanes but that’s an unlikely situation.
There is a single byte change in the EDID between the 201388 and CAC-1085. That shouldn’t happen if I’m using the same connection on the same display for each. It appears the single bit change is in the second byte of the AMD vendor specific data block – maybe it’s just a flag that signals HDR/not HDR? I can’t seem to find info on the AMD vendor specific data block contents.