The following bug was just posted: i915 failures with recent chips and docking stations https://bugs.freedesktop.org/show_bug.cgi?id=100954 and follows up this thread https://lists.freedesktop.org/archives/intel-gfx/2017-May/127350.htmlin the intel-gfx mailing list. Per suggestion of Jani Nikula of Intel, I'm also posting a brief summary to this list.

I am the developer of ddcutil http://www.ddcutil.com, a Linux utility that manages monitor settings using DDC/CI. I am seeing a pattern of user error reports in which I2C communication is not working when a system with a recent Intel chip, using the i915 driver, is plugged into a docking station.

ddcutil looks for displays by examining all non-SMBus /dev/i2c devices on the system. If checks for the presence of slave address x50 and x37. If they exist it tries to read the EDID on x50 and a Virtual Control Panel feature value on x37. Examining one of the user logs, I see that two /dev/i2c-n devices have udev sysattr name DPMST. When ddcutil probes those /dev/i2c devices, slave addresses x50 and x37 appear active, but reading the EDID fails.

Per Jani Nikola, this is likely a bug in the core drm DP MST code.

Sanford Rockowitz