Created attachment 91154 [details] [review]
possible fix

(In reply to comment #34)
> 
> $ lspci -nn | grep 'VGA'
> 00:01.0 VGA compatible controller [0300]: Advanced Micro Devices, Inc.
> [AMD/ATI] Sumo [Radeon HD 6480G] [1002:9649]
> 
> [  188.599127] [drm] initializing kernel modesetting (SUMO 0x1002:0x9649
> 0x17AA:0x21EA).
> 
> 
> Thus, is's SUMO isn't it? Should I try to use "max_hw_contexts = 4" for the
> SUMO case or that's unreasonable?

Your chip was misclassified.  The attached patch should fix it.