Created attachment 127625 [details]
dmesg with drm.debug=0x1e

System architecture: x86_64
Kernel version: 4.9.0-rc2+ (drm-intel-nightly, latest checkout (commit bbb625b))
Linux distribution: Gentoo Linux
Machine: Acer TravelMate B116-M-P16S
Processor: Intel Pentium N3700
Display connector: HDMI

I have connected an external monitor to my laptop using HDMI, internal LCD is disabled. I am using the default settings that the screen is disabled after some time of inactivity. Sometimes, when I press a key or move the mouse to turn back on the screen, I only see a white screen (sometimes a different color, but mostly white). Starting programs using a keyboard shortcut does not help to get back the desktop.
My workaround is to call a script using a keyboard shortcut that turns off HDMI output (xrandr --output HDMI1 --off) and enables the internal LCD (xrandr --output eDP1 --auto --primary). After that I call a different script that turns off the internal LCD and switches back to HDMI output. Sometimes I have to call the scripts multiple times until I can resume work.
If the problem happens the kernel log shows:

[drm:intel_set_cpu_fifo_underrun_reporting [i915]] *ERROR* pipe A underrun
[drm:intel_cpu_fifo_underrun_irq_handler [i915]] *ERROR* CPU pipe A FIFO underrun

I have attached the dmesg output with drm.debug=0x1e.