I2C failures with recent laptops and docking stations

[Sanford Rockowitz]

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.html>in 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



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