DDC communication failed (docking station + HDMI) about ddcutil HOT 5 OPEN

I'm pretty sure that what has changed is that the video connection for the X270 dock is standard DisplayPort, whereas the new T14/Lenovo dock combination uses USB-C alt mode to transmit the DisplayPort signal, and the I²C signalling is being lost.

Note that when a docking station is used the connection to the dock is always DP Multi-Stream Transport (MST). If there is a HDMI connector on the dock the DP signal is converted to HDMI.

"Phantom display" is a way of describing the phenomenon of a display appearing at two different /dev/i2c devices. Typically only one actually supports DDC. ddcutil reports the invalid /dev/i2c device as "phantom".

See Docking Connected Displays, The same DisplayPort connected monitor appears twice in the output of ddcutil detect. and also this extensive freedesktop.org bug report thread.

This warrants a [i915 bug report] ( https://drm.pages.freedesktop.org/intel-docs/how-to-file-i915-bugs.html). Let me know when you've filed it and I'll add myself to the CC list. But I wouldn't expect a fix any time soon.

from ddcutil.

This warrants a [i915 bug report] ( https://drm.pages.freedesktop.org/intel-docs/how-to-file-i915-bugs.html). Let me know when you've filed it and I'll add myself to the CC list. But I wouldn't expect a fix any time soon.

I don't get it, what is the i915 bug that I should file? Is the communication between ddcutil and the HDMI monitor failing due to a bug in the i915 driver?

from ddcutil.

In your new setup, the T14 <-> Docking Station communication is by a USB Type-C connector, operating in display port alt mode. I²C signalling is handled over the DP Aux channel. The dock is a DP Multi-Stream Transport device. The single DP stream from laptop to dock actually carries several DP streams, which are split out at the dock. For the HDMI connector on the dock, the dock converts DP signalling to HDMI signalling. In particular, the I²C signal that is "multiplexed" over the the DP Aux channel becomes simple I²C signalling on the appropriate pins of the HDMI connector. Somewhere along this chain the I²C signal is being lost.

from ddcutil.

Thanks for the explanation, but I still don't know what would I file in such a bug report to the i915 driver developers.
And they will just brush it off and say that the dock is defective or that the dock doesn't implement that function.

from ddcutil.

The docking station model is a Lenovo Universal Thunderbolt 4 (40b0) (I just updated the issue to reflect this).
I tried updating the firmware to the latest version, it didn't help.
I also tried connecting both monitors via DP, but still only one of them is detected.
I'm about to buy a 3rd monitor, but I'm wondering if I should get one that supports USB TB instead of just DP or HDMI.
My aim is to be able to control 3 monitors, so I can easily change the brightness in a script, instead of using their menu system one by one, with the controls in the back. Is there any hope for this?

from ddcutil.