我刚刚安装了一些更新并重新启动，之后 Ubuntu 无法检测到外部显示器。它通过 HDMI 连接。我该怎么办？它没有出现在显示设置中。我使用笔记本电脑，内置显示器正在工作并被检测到，但今天更新后外部显示器停止工作。这台电脑是戴尔 Latitude E7450。
$ lspci | grep VGA
00:02.0 VGA compatible controller: Intel Corporation Broadwell-U Integrated Graphics (rev 09)
~$ dmesg | grep -E 'VGA|HDMI'
[ 0.882972] fb0: VESA VGA frame buffer device
[ 5.718803] fb: switching to inteldrmfb from VESA VGA
[ 5.719040] [drm] Replacing VGA console driver
[ 6.224466] input: HDA Intel HDMI HDMI/DP,pcm=3 as /devices/pci0000:00/0000:00:03.0/sound/card0/input16
[ 6.224530] input: HDA Intel HDMI HDMI/DP,pcm=7 as /devices/pci0000:00/0000:00:03.0/sound/card0/input17
[ 6.224590] input: HDA Intel HDMI HDMI/DP,pcm=8 as /devices/pci0000:00/0000:00:03.0/sound/card0/input18
$ dmesg | grep -iE 'fail|error|skip'
[ 0.000022] Calibrating delay loop (skipped), value calculated using timer frequency.. 5187.65 BogoMIPS (lpj=10375316)
[ 3.285636] psmouse serio1: Failed to enable mouse on isa0060/serio1
[ 4.077454] EXT4-fs (sda1): re-mounted. Opts: errors=remount-ro
[ 5.849972] init: failsafe main process (612) killed by TERM signal
[ 7.787412] systemd-logind[854]: Failed to start unit [email protected]: Unknown unit: [email protected]
[ 7.787417] systemd-logind[854]: Failed to start user service: Unknown unit: [email protected]
[ 8.418720] systemd-logind[854]: Failed to start unit [email protected]: Unknown unit: [email protected]
[ 8.418725] systemd-logind[854]: Failed to start user service: Unknown unit: [email protected]
[ 9.170491] vboxdrv: module verification failed: signature and/or required key missing - tainting kernel
[ 16.373837] systemd-logind[854]: Failed to start unit [email protected]: Unknown unit: [email protected]
[ 16.373842] systemd-logind[854]: Failed to start user service: Unknown unit: [email protected]
$ xrandr
Screen 0: minimum 8 x 8, current 1920 x 1080, maximum 32767 x 32767
eDP1 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 309mm x 173mm
1920x1080 60.0*+ 59.9 48.0
1680x1050 60.0 59.9
1600x1024 60.2
1400x1050 60.0
1280x1024 60.0
1440x900 59.9
1280x960 60.0
1360x768 59.8 60.0
1152x864 60.0
1024x768 60.0
800x600 60.3 56.2
640x480 59.9
DP1 disconnected (normal left inverted right x axis y axis)
HDMI1 disconnected (normal left inverted right x axis y axis)
VIRTUAL1 disconnected (normal left inverted right x axis y axis)
更新
以下步骤解决了该问题：从 USB 启动 Ubuntu 14 live。然后重新启动到本机安装的 Ubuntu，外部屏幕即可正常工作。