I have a Precision 7720 with a Quadro P4000 card and a Vizio E48U-D0 4k display that I've been running fine at 3840x2160 using HDMI on the laptop's local HDMI port. When I plug it into the HDMI port on the TB16, the display will only go up to 1920x1080@60. The only resolutions available appear to be the standard ones from vga thru 1920x1080, which leads me to believe there is a EDID issue happening between the dock and the video card.
FWIW, I've reinstalled all the BIOS/Thunderbolt Firmware/Controller/Driver as well as reloaded the Intel and Nvidia drivers from scratch with all current drivers on the support page. I've also tried all the combinations of switchable graphics settings in the bios. Same results.
The TB16 HDMI port is HDMI 1.4a and doesn't support 4k @ 60hz:
What refresh rate were you using when connected directly to the 7720?
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The monitor can do all the usual 3840x2160 24/25/29.97/30p as well as 60p. But it's not giving me the option at all for anything higher than 1920x1080 in the Nvidia resolution panel. The panel reports the monitor as 1920x1080 (native) incorrectly...
Trying to create a custom timing manually in the with the exact same numbers as when it is connected to the laptop's HDMI port fails to connect. Same result when capturing an Edid with system topology from the HDMI port on laptop @29.97, and applying it to the dock's HDMI port.
I did a test with a Barco E2 system, and I did get both the HDMI and DP out simultaneously at firstname.lastname@example.org so I really suspect something in the edid chain is broken. I've done a full CommandUpdate clean advanced driver restore,and that didn't change anything.
Might just be the monitor's edid is bad just enough to cause it to fail.. but still can't determine why.