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<b><a class="bz_bug_link
bz_status_NEEDINFO "
title="NEEDINFO - [snb DP-to-DVI dongle] miniDP to external monitor with DVI-D via miniDP to DVI-D adapter and blank screen"
href="https://bugs.freedesktop.org/show_bug.cgi?id=106596#c7">Comment # 7</a>
on <a class="bz_bug_link
bz_status_NEEDINFO "
title="NEEDINFO - [snb DP-to-DVI dongle] miniDP to external monitor with DVI-D via miniDP to DVI-D adapter and blank screen"
href="https://bugs.freedesktop.org/show_bug.cgi?id=106596">bug 106596</a>
from <span class="vcard"><a class="email" href="mailto:genpur@o2.pl" title="genpur@o2.pl">genpur@o2.pl</a>
</span></b>
<pre>Kernel (4.16.10) has been built with "CONFIG_DRM_DP_AUX_CHARDEV=y"
configuration.
Additionally I have also checked few kernels from 4.4 up to 4.16 and all of
them have been built with this configuration.
Device /dev/drm_dp_aux0 exists in my system :
LC_MESSAGES=en_UK.UTF-8 ls -lah /dev/drm_dp_aux*
crw------- 1 root root 243, 0 May 24 2018 /dev/drm_dp_aux0
When adapter is disconnected :
LC_MESSAGES=en_UK.UTF-8 sudo dd if=/dev/drm_dp_aux0 bs=1 skip=$((0x200))
count=1 | od -tx1
dd: error reading '/dev/drm_dp_aux0': Connection timed out
0+0 records in
0+0 records out
0 bytes copied, 0.0804776 s, 0.0 kB/s
0000000
When adapter is connected :
LC_MESSAGES=en_UK.UTF-8 sudo dd if=/dev/drm_dp_aux0 bs=1 skip=$((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
0000001
Another info for you :
I have discovered that when system is started with kernel <= 4.10 with kernel
parameter "video=DP-1:D" then it doesn't matter if adapter was plugged or
unplugged while booting. External monitor is activated anytime adapter is
plugged to the laptop. All the resolutions/refresh rates are available
according to technical specification of the monitor, so EDID seems to be read
corectly and seems to be not corrupted.
In this case the command "dd if=/dev/drm_dp_aux0 bs=1 skip=$((0x200)) count=1 |
od -tx1" also returns "00".</pre>
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