[Intel-gfx] i915 I2C failures with recent chips and docking stations

[Sanford Rockowitz]

I am the author of ddcutil (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.  Communication works when the video cable is plugged
directly into the laptop.   There also seems to be a correlation with
whether a DisplayPort cable is being used (i.e. the I2C-over-aux path is
being executed), though this correlation is not as clear.

I'm not sure how best to go about reporting these issues.  The usual bug
reporting mechanism asks for detailed information about a specific
system, which I don't have.   What I do have is the ability to see a
pattern.

Because I2C communication is so fragile (dependencies on hardware
quirks, drivers, etc), ddcutil (which is written in C) has an extensive
subsystem devoted to remotely diagnosing user configurations.  It
already reports DDC (i.e. I2C) communication errors, and has interfaces
to udev, libdrm, and x11 libraries.  If there is some i915 specific code
I could add to refine the diagnosis, I would be happy to add that. 
(Note: ddcutil is entirely a user space program, using the i2c-dev
interface.)

Suggestions on how to proceed appreciated.

Sanford Rockowitz