Sometimes my external monitor (Acer X223W) stays black (powersave mode) when resuming from suspend. I'm using a HP ProBook 6465b (Llano/Sumo, AMD A6-APU, Radeon HD6520G). The Monitor is connected to the DVI port of my HP docking station (P/N: VB044AV).

My Desktop Environment (GNOME 3) still thinks it's connected, so some windows aren't visible anymore.

Workarounds:
- Press WIN+P to disable the second monitor and use the Laptop screen only
- Power off the external monitor and then switch it on again. It then starts to work again.

Some more information:
$ dmesg | tail -n 2
[ 7977.467770] [drm:radeon_dp_link_train [radeon]] *ERROR* displayport link status failed
[ 7977.467815] [drm:radeon_dp_link_train [radeon]] *ERROR* clock recovery failed

$ uname -a
Linux jan-laptop 4.2.5-1-ARCH #1 SMP PREEMPT Tue Oct 27 08:13:28 CET 2015 x86_64 GNU/Linux

$ glxinfo | grep OpenGL | grep string
OpenGL vendor string: X.Org
OpenGL renderer string: Gallium 0.4 on AMD SUMO (DRM 2.43.0, LLVM 3.7.0)
OpenGL core profile version string: 3.3 (Core Profile) Mesa 11.0.5
OpenGL core profile shading language version string: 3.30
OpenGL version string: 3.0 Mesa 11.0.5
OpenGL shading language version string: 1.30
OpenGL ES profile version string: OpenGL ES 3.0 Mesa 11.0.5
OpenGL ES profile shading language version string: OpenGL ES GLSL ES 3.00

$ xrandr
Screen 0: minimum 320 x 200, current 3046 x 1050, maximum 8192 x 8192
VGA-0 disconnected (normal left inverted right x axis y axis)
LVDS connected primary 1366x768+0+0 (normal left inverted right x axis y axis) 309mm x 173mm
   1366x768      60.10*+
   1280x720      59.86  
   1152x768      59.78  
   1024x768      59.92  
   800x600       59.86  
   848x480       59.66  
   720x480       59.71  
   640x480       59.38  
DisplayPort-0 disconnected (normal left inverted right x axis y axis)
DisplayPort-1 disconnected (normal left inverted right x axis y axis)
DisplayPort-2 connected 1680x1050+1366+0 (normal left inverted right x axis y axis) 474mm x 296mm
   1680x1050     59.95*+
   1600x1200     60.00  
   1400x1050     59.95  
   1280x1024     75.02  
   1440x900      74.98    59.90  
   1280x960      60.00  
   1360x768      59.80  
   1152x864      75.00  
   1024x768      75.08    70.07    60.00  
   832x624       74.55  
   800x600       72.19    75.00    60.32    56.25  
   640x480       75.00    72.81    66.67    60.00  
   720x400       70.08  

$ cat /proc/cpuinfo
processor       : 0
vendor_id       : AuthenticAMD
cpu family      : 18
model           : 1
model name      : AMD A6-3410MX APU with Radeon(tm) HD Graphics
stepping        : 0
microcode       : 0x3000027
cpu MHz         : 1000.000
cache size      : 1024 KB
physical id     : 0
siblings        : 4
core id         : 0
cpu cores       : 4
apicid          : 0
initial apicid  : 0
fpu             : yes
fpu_exception   : yes
cpuid level     : 6
wp              : yes
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt pdpe1gb rdtscp lm 3dnowext 3dnow constant_tsc rep_good nopl nonstop_tsc extd_apicid aperfmperf pni monitor cx16 popcnt lahf_lm cmp_legacy svm extapic cr8_legacy abm sse4a misalignsse 3dnowprefetch osvw ibs skinit wdt arat cpb hw_pstate npt lbrv svm_lock nrip_save pausefilter vmmcall
bugs            : fxsave_leak sysret_ss_attrs
bogomips        : 3195.26
TLB size        : 1536 4K pages
clflush size    : 64
cache_alignment : 64
address sizes   : 40 bits physical, 48 bits virtual
power management: ts ttp tm stc 100mhzsteps hwpstate cpb

[... and 3 more times]