CAC-1085 and issues with current nVidia driver (Club3D or nVidia?)

5 2 days ago

Greetings.

Some users on this forum are reporting issues with the CAC-1085 adapter and recent drivers from nVidia. It would be interesting to know if this is common, and what cards does it happen with.

The reason for this is, after spending last night trying to enable 120Hz in Linux with the most recent driver from nVidia (460.32.03), I stumbled across this error in my logs related to 3840×[email protected] (reported by the driver, apparently):

[ 2358.878] (WW) NVIDIA(GPU-0): Mode is rejected: DP Bandwidth check failed.
[ 2358.878] (WW) NVIDIA(GPU-0): GPU extended capability check failed.

The question is, why does the bandwidth check fail? Does the driver fail to negotiate with the device because it ignores that the adapter is using DSC (without which the bandwidth would probably be insuffcient)? Can this be affected with firmware or is it a 100% nVidia thing? I’m not familiar enough with the subject, but hopefully someone is 😀

And does this affect all nVidia users trying to use resolution/refresh/bit depth combinations with “too high” bandwidth requirements through the adapter?

I’m using the adapter with a Asus Strix 2080Ti, and can not enable these settings.

A working solution to this would be great, with the lack on any actual availability of modern GPUs.

 
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2 2 days ago

Hi Suti,

I’ve seen many problems like this.

I assume you are using an LG C9? If you downgrade to driver 456.38 you will be able to get 4k120Hz. Not sure if the drivers are different for Linux but I believe drivers before November 2020 should work.

I wonder if you get any other error logs via Linux when it doesn’t give you the 4k120Hz. Everyone is wondering why the drivers of Nvidia “block”  the CAC-1085.

23 hours ago

Close, the CX. 

Those two are the only errors that seem to pop up. Rolling back to 440 everything works just fine. 

It is very odd that nVidia would disable a feature just like that. 

7 hours ago

Yeah it is very odd..

But a very bold assumption would be they do that because they want to sell their RTX3000 series and not want to have the RTX2000 series be able to do 4K120Hz by a third party adapter…. But we will never know I guess.

 

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1 2 days ago

Club-3d can’t fix this. The adapter reports support for DSC but the driver doesn’t support DSC? Talk to the people that created the driver. Does the Linux Nvidia driver support DSC? AMD added DSC support for Linux last year.

I have a question about the functioning of the CAC-1085 and HDMI 2.1. Is there a difference between HDMI 2.0 and 2.1 cables that the adapter can detect or does it have to test the four lanes for signal quality? Does the CAC-1085 change any info (DPCD or EDID) reported to the host depending on the connection or is all info expected to be in the EDID?

7 hours ago

Hi Joevt,

HDMI 2.0 is a TMDS signal where the HDMI 2.1 is a FRL signal. Which gets detected by the adapter. If you plug in an HDMI 2.0 cable it wont be able to push a 4k120hz signal. All info is in  the EDID.

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