I installed Linux Mint 17 MATE recently. My laptop (ThinkPad X1 Carbon) works fine by itself, and also with an external screen attached. Brightness control keys work.
However, once I detach the external screen, the laptop screen goes really dark (maybe 5% of regular brightness or so); on bright days it almost looks like turned off. The brightness control keys stop working when detaching the external screen. I tried various things (`xrandr --output LVDS1 --brightness 1` or `echo 15 > /sys/class/backlight/acpi_video0/brightness`), but could not find anything that solves this. Suspending and resuming also does not help.
When I attach the secondary screen again, the laptop screen behaves normally again. However, I sometimes have to work with external screen and sometimes without, so using an external screen is not always a solution. In these cases, only rebooting helps ...
Here's the output of lspci:
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$ lspci
00:00.0 Host bridge: Intel Corporation 3rd Gen Core processor DRAM Controller (rev 09)
00:02.0 VGA compatible controller: Intel Corporation 3rd Gen Core processor Graphics Controller (rev 09)
00:14.0 USB controller: Intel Corporation 7 Series/C210 Series Chipset Family USB xHCI Host Controller (rev 04)
00:16.0 Communication controller: Intel Corporation 7 Series/C210 Series Chipset Family MEI Controller #1 (rev 04)
00:16.3 Serial controller: Intel Corporation 7 Series/C210 Series Chipset Family KT Controller (rev 04)
00:1a.0 USB controller: Intel Corporation 7 Series/C210 Series Chipset Family USB Enhanced Host Controller #2 (rev 04)
00:1b.0 Audio device: Intel Corporation 7 Series/C210 Series Chipset Family High Definition Audio Controller (rev 04)
00:1c.0 PCI bridge: Intel Corporation 7 Series/C210 Series Chipset Family PCI Express Root Port 1 (rev c4)
00:1c.1 PCI bridge: Intel Corporation 7 Series/C210 Series Chipset Family PCI Express Root Port 2 (rev c4)
00:1d.0 USB controller: Intel Corporation 7 Series/C210 Series Chipset Family USB Enhanced Host Controller #1 (rev 04)
00:1f.0 ISA bridge: Intel Corporation QS77 Express Chipset LPC Controller (rev 04)
00:1f.2 SATA controller: Intel Corporation 7 Series Chipset Family 6-port SATA Controller [AHCI mode] (rev 04)
00:1f.3 SMBus: Intel Corporation 7 Series/C210 Series Chipset Family SMBus Controller (rev 04)
02:00.0 System peripheral: Ricoh Co Ltd PCIe SDXC/MMC Host Controller (rev 07)
03:00.0 Network controller: Intel Corporation Centrino Advanced-N 6205 [Taylor Peak] (rev 96)
Thanks,
Peter