[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 #12 from genpur at o2.pl ---
(In reply to Jani Nikula from comment #9)
> (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.

dd if=/dev/drm_dp_aux0 bs=1 skip=$((0x80)) count=16 | od -tx1 returns
always(*):
0000000 0b 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

(*)always - means for all the cases I have checked when adapter was plugged in
to PC and : kernels 4.16, 4.10, monitor undetected or other TV display
detected.

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