我有一个外部显示器，通过 HDMI 端口连接到外部显示器的 DVI 输入。监视器永远不会被检测到。我使用 Intel 视频驱动程序运行 ubuntu 17.1； lspci -v 发现：
00:02.0 VGA compatible controller: Intel Corporation HD Graphics 520 (rev 07) (prog-if 00 [VGA controller])
Subsystem: Toshiba America Info Systems HD Graphics 520
Flags: bus master, fast devsel, latency 0, IRQ 126
Memory at c0000000 (64-bit, non-prefetchable) [size=16M]
Memory at b0000000 (64-bit, prefetchable) [size=256M]
I/O ports at 3000 [size=64]
[virtual] Expansion ROM at 000c0000 [disabled] [size=128K]
Capabilities: <access denied>
Kernel driver in use: i915
Kernel modules: i915
显示器是三星mega型号BX2335，笔记本电脑是东芝tecra-z40-c，内核是4.13.0-32-generic。 xrandr 显示：
jeremy@jeremy-TECRA-Z40-C-pipl:~/Downloads$ xrandr
Screen 0: minimum 320 x 200, current 1920 x 1080, maximum 8192 x 8192
eDP-1 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 309mm x 174mm
1920x1080 60.02*+ 59.93
1680x1050 59.95 59.88
....<a bunch of resolutions>....
320x240 60.05
DP-1 disconnected (normal left inverted right x axis y axis)
HDMI-1 disconnected (normal left inverted right x axis y axis)
DP-2 disconnected (normal left inverted right x axis y axis)
HDMI-2 disconnected (normal left inverted right x axis y axis)
第一个屏幕是笔记本电脑屏幕。 xorg.conf 看起来没问题：
jeremy@jeremy-TECRA-Z40-C-pipl:~/Downloads$ more /usr/share/doc/xserver-xorg-video-intel/xorg.conf
Section "Device"
Identifier "Intel"
Driver "intel"
# Option "AccelMethod" "uxa"
EndSection
显示器控制面板未检测到外接显示器。我重新启动，运行 apt update、upgrade 和 dist-upgrade，并尝试了驱动程序升级（没有找到任何内容，我也没有强制执行任何操作）。