Comment 22 for bug 1242115

BTW, before I upgraded to trusty it was working fine (12.04). Here's the output of /usr/bin/intel_reg_checker :

MI_MODE (0x209c): 0x00000000
  (bit 14) OK: Async Flip Performance mode
  (bit 13) OK: Flush Performance Mode
  (bit 12) FAIL: MI_FLUSH enable must be set
  (bit 6) FAIL: Vertex Shader Timer Dispatch Enable must be set
GFX_MODE (0x229c): 0x00000000
  (bit 13) PERF: Flush TLB Invalidation Mode should be set
GT_MODE (0x7008): 0x00000000
CACHE_MODE_0 (0x7000): 0x00000000
  (bit 15) OK: Sampler L2 Disable
  (bit 9) PERF: Sampler L2 TLB Prefetch Enable should be set
  (bit 8) OK: Depth Related Cache Pipelined Flush Disable
  (bit 5) OK: STC LRA Eviction Policy
  (bit 4) OK: RCC LRA Eviction Policy
  (bit 3) OK: Hierarchical Z Disable
  (bit 0) OK: Render Cache Operational Flush
CACHE_MODE_1 (0x7004): 0x00000000
  (bit 13) OK: STC Address Lookup Optimization Disable
  (bit 12) OK: HIZ LRA Eviction Policy
  (bit 11) OK: DAP Instruction and State Cache Invalidate
  (bit 10) OK: Instruction L1 Cache and In-Flight Queue Disable
  (bit 9) OK: Instruction L2 Cache Fill Buffers Disable
  (bit 6) OK: Pixel Backend sub-span collection Optimization Disable
  (bit 5) OK: MCS Cache Disable
  (bit 4) OK: Data Disable
  (bit 1) OK: Instruction and State L2 Cache Disable
  (bit 0) OK: Instruction and State L1 Cache Disable
FF_SLICE_CHICKEN (0x2088): 0x00000000
           OK: chicken bits unset
3D_CHICKEN3 (0x2090): 0x00000000
           OK: chicken bits unset
FF_SLICE_CS_CHICKEN1 (0x20e0): 0x00000000
           OK: chicken bits unset
FF_SLICE_CS_CHICKEN2 (0x20e4): 0x00000000
           OK: chicken bits unset
FF_SLICE_CS_CHICKEN3 (0x20e8): 0x00000000
           OK: chicken bits unset
COMMON_SLICE_CHICKEN1 (0x7010): 0x00000000
           OK: chicken bits unset
COMMON_SLICE_CHICKEN2 (0x7014): 0x00000000
           OK: chicken bits unset
WM_CHICKEN (0x5580): 0x00000000
           OK: chicken bits unset
HALF_SLICE_CHICKEN (0xe100): 0x00000000
           OK: chicken bits unset
HALF_SLICE_CHICKEN2 (0xe180): 0x00000000
           OK: chicken bits unset
ROW_CHICKEN (0xe4f0): 0x00000000
           OK: chicken bits unset
ROW_CHICKEN2 (0xe4f4): 0x00000000
           OK: chicken bits unset
ECOSKPD (0x21d0): 0x00000000
           OK: chicken bits unset

Not sure if that helps. I tried kernels 3.8, 3.12 and 3.13, no difference.