How do I debug DisplayPort hotplug detection?

Marius Gedminas marius at
Tue Jan 14 04:16:03 PST 2014

I've got a Lenovo ThinkPad X220 with Intel graphics.  When I plug in a
VGA cable, my desktop automatically notices it and reconfigures the
desktop.  When I use a DisplayPort -> DVI cable, this doesn't happen and
I have to run xrandr by hand (with no arguments is fine, merely querying
RANDR makes the driver notice I plugged in the cable).

So far I filed an Ubuntu bug[1], tried to upgrade my BIOS (well, UEFI, I
suppose), tried the latest upstream kernel.  No luck.


How can I debug this?  Which component is supposed to detect these
events -- the kernel driver or the userspace bits in  If it's the
kernel, what's the mechanism it uses to notify userspace about that
event?  How does X notify clients about newly appearing monitors?  What
command-line tools are there to watch for these events (not) showing up?
What's the appropriate upstream for this bug?

Marius Gedminas
Well, there's a quantum computer that can factor 15, so those of you using
4-bit RSA should worry.
        -- Bruce Schneier
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