[Bug 106596] [snb DP-to-DVI dongle] miniDP to external monitor with DVI-D via miniDP to DVI-D adapter and blank screen

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=106596

--- Comment #9 from Jani Nikula <jani.nikula at intel.com> ---
(In reply to genpur from comment #7)
> When adapter is disconnected :
> LC_MESSAGES=en_UK.UTF-8 sudo dd if=/dev/drm_dp_aux0 bs=1 skip=$((0x200))
> count=1 | od -tx1
> dd: error reading '/dev/drm_dp_aux0': Connection timed out

As expected.

> When adapter is connected :
> LC_MESSAGES=en_UK.UTF-8 sudo dd if=/dev/drm_dp_aux0 bs=1 skip=$((0x200))
> count=1 | od -tx1
> 1+0 records in
> 1+0 records out
> 1 byte copied, 0.000779381 s, 1.3 kB/s
> 0000000 00

That 00 tells us the adapter thinks there are no displays connected to it. That
should dynamically change depending on the display being plugged/unplugged into
the other end of the cable, and generate a hotplug event. Please double check
with a display, and ensure it's on.

How about

dd if=/dev/drm_dp_aux0 bs=1 skip=$((0x80)) count=16 | od -tx1

Ville, perhaps we should only trust sink count if downstream port is hotplug
capable, and assume 1 otherwise? Regardless of what the spec says. See the sink
count handling in intel_dp_detect_dpcd() - we never reach that if sink count is
0 in intel_dp_get_dpcd(). Not very consistent.

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