No picture through Thunderbolt 4

Continuing my quest for a USB-C dock... I've made a lot of progress, and right now I'm getting good keyboard, mouse, audio, network, and power through my USB-C connection to my laptop with Thunderbolt 4. However, I still do not have video. But oddly enough, my laptop thinks everything is hunky dory. (Almost. I later found something interesting in dmesg.)

I have two 1920x1080 monitors connected to the two HDMI ports on my USB-C dock. Whenever I connect the dock, my laptop automatically reconfigures the display to accommodate the two extra monitors. They show up just fine in xrandr. Everything seems perfect, except for the fact that the monitors themselves stay black.

When I connect either monitor directly to the laptop's built-in HDMI port, it roars to life, proving that the monitors and the cables are good.

How can it be that the laptop is able to communicate with the monitors to get their model numbers, supported resolutions, etc., yet still the monitors themselves act as if they're not connected?

So far I tried KDE Plasma and MATE.

I'm running slackware64-current Fri Feb 19 20:53:50 UTC 2021.

Right this second, for the sake of having anything at all, I have one of the monitors connected directly to the laptop. The other is still connected to the dock, though turned off in software (xrandr --off) so that windows don't end up getting mapped to it. I also have the laptop internal screen disabled.

Here's the interesting thing I'm seeing in dmesg:

Code:[ 2113.952704] usb 3-2.5: new high-speed USB device number 10 using xhci_hcd
[ 2114.030772] usb 3-2.5: New USB device found, idVendor=291a, idProduct=8380, bcdDevice= 0.01
[ 2114.030774] usb 3-2.5: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[ 2114.030775] usb 3-2.5: Product: Anker USB-C Hub Device
[ 2114.030776] usb 3-2.5: Manufacturer: Anker
[ 2114.030776] usb 3-2.5: SerialNumber: 0000000000000001
[ 2114.087291] i915 0000:00:02.0: [drm] *ERROR* Link Training UnsuccessfulThat last line is repeated a few dozen times.

I wonder if this is relevant. My next step is to try kernel 5.11. I just kicked off a download of the source tree, will take a while to complete.

Some more information:

Code:$ lsusb
Bus 004 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
Bus 003 Device 004: ID 0bda:0129 Realtek Semiconductor Corp. RTS5129 Card Reader Controller
Bus 003 Device 003: ID 13d3:56a2 IMC Networks USB2.0 HD UVC WebCam
Bus 003 Device 010: ID 291a:8380 Anker Anker USB-C Hub Device
Bus 003 Device 009: ID 13ba:0018 PCPlay Barcode PCP-BCG4209
Bus 003 Device 005: ID 0bda:4c07 Realtek Semiconductor Corp. TX-Hifi Type_C Audio
Bus 003 Device 002: ID 2109:2817 VIA Labs, Inc. USB2.0 Hub
Bus 003 Device 006: ID 8087:0026 Intel Corp.
Bus 003 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 002 Device 003: ID 05e3:0749 Genesys Logic, Inc. USB3.0 Card Reader
Bus 002 Device 002: ID 2109:0817 VIA Labs, Inc. USB3.0 Hub
Bus 002 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
$ lspci
0000:00:00.0 Host bridge: Intel Corporation 11th Gen Core Processor Host Bridge/DRAM Registers (rev 01)
0000:00:02.0 VGA compatible controller: Intel Corporation TigerLake GT2 [Iris Xe Graphics] (rev 01)
0000:00:04.0 Signal processing controller: Intel Corporation TigerLake-LP Dynamic Tuning Processor Participant (rev 01)
0000:00:07.0 PCI bridge: Intel Corporation Tiger Lake-LP Thunderbolt 4 PCI Express Root Port #0 (rev 01)
0000:00:08.0 System peripheral: Intel Corporation Device 9a11 (rev 01)
0000:00:0a.0 Signal processing controller: Intel Corporation Tiger Lake-LP Telemetry Aggregator (rev 01)
0000:00:0d.0 USB controller: Intel Corporation Tiger Lake-LP Thunderbolt 4 USB Controller (rev 01)
0000:00:0d.2 USB controller: Intel Corporation Tiger Lake-LP Thunderbolt 4 NHI #0 (rev 01)
0000:00:0e.0 RAID bus controller: Intel Corporation Volume Management Device NVMe RAID Controller
0000:00:14.0 USB controller: Intel Corporation Tiger Lake-LP USB 3.2 Gen 2x1 xHCI Host Controller (rev 20)
0000:00:14.2 RAM memory: Intel Corporation Tiger Lake-LP Shared SRAM (rev 20)
0000:00:14.3 Network controller: Intel Corporation Wi-Fi 6 AX201 (rev 20)
0000:00:15.0 Serial bus controller [0c80]: Intel Corporation Tiger Lake-LP Serial IO I2C Controller #0 (rev 20)
0000:00:15.1 Serial bus controller [0c80]: Intel Corporation Tiger Lake-LP Serial IO I2C Controller #1 (rev 20)
0000:00:16.0 Communication controller: Intel Corporation Tiger Lake-LP Management Engine Interface (rev 20)
0000:00:1d.0 System peripheral: Intel Corporation Device 09ab
0000:00:1e.0 Communication controller: Intel Corporation Tiger Lake-LP Serial IO UART Controller #0 (rev 20)
0000:00:1e.2 Serial bus controller [0c80]: Intel Corporation Device a0aa (rev 20)
0000:00:1f.0 ISA bridge: Intel Corporation Tiger Lake-LP LPC Controller (rev 20)
0000:00:1f.3 Audio device: Intel Corporation Tiger Lake-LP Smart Sound Technology Audio Controller (rev 20)
0000:00:1f.4 SMBus: Intel Corporation Tiger Lake-LP SMBus Controller (rev 20)
0000:00:1f.5 Serial bus controller [0c80]: Intel Corporation Tiger Lake-LP SPI Controller (rev 20)
10000:e0:1d.0 PCI bridge: Intel Corporation Tiger Lake-LP PCI Express Root Port #9 (rev 20)
10000:e1:00.0 Non-Volatile memory controller: Intel Corporation SSD 660P Series (rev 03)
$ xrandr
Screen 0: minimum 320 x 200, current 1920 x 1080, maximum 16384 x 16384
eDP-1 connected (normal left inverted right x axis y axis)
1920x1080 60.01 + 60.01 59.97 59.96 59.93
[...]
HDMI-1 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 528mm x 297mm
1920x1080 60.00*+ 74.97 50.00 59.94
[...]
DP-1 disconnected (normal left inverted right x axis y axis)
DP-1-1 disconnected (normal left inverted right x axis y axis)
DP-1-2 connected (normal left inverted right x axis y axis)
1920x1080 60.00 + 74.97 50.00 59.94
[...]
DP-1-3 disconnected (normal left inverted right x axis y axis)