[Intel-gfx] Skylake Linux 4.2 FIFO underruns

[Daniel Drake]

Hi,

Using Linux 4.2 on a new Skylake laptop, I frequently see the internal
display go momentarily black, with a coinciding message in the logs:

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

Testing linus master, the problem is gone. Does anyone know what
commit might have fixed this? I would like to backport to 4.2.

I already tried cherry-picking the patches that mention skl that apply
trivially, and while that fixed some GPU hangs I was seeing, the FIFO
underrun is still present with 4.2 plus:

 drm/i915/skl: Retrieve the Rpe value from Pcode
 drm/i915/skl: WaIgnoreDDIAStrap is forever, always init DDI A
 drm/i915/skl WaDisableSbeCacheDispatchPortSharing
 drm/i915:skl: Add WaEnableGapsTsvCreditFix
 drm/i915/skl: Restrict the ring frequency table programming to SKL
 drm/i915/skl: Ring frequency table programming changes
 drm/i915/skl: Don't warn if reading back DPLL0 is disabled
 drm/i915: Move WaBarrierPerformanceFixDisable:skl to skl code from chv code
 drm/i915/skl: Don't try to store the wrong central frequency
 drm/i915/skl: Remove unnecessary () used with abs_diff()
 drm/i915/skl: Remove unnecessary () used with div_u64()
 drm/i915/skl: Factor out computing the DPLL paramaters from the dividers
 drm/i915/skl: Use a more idomatic early return
 drm/i915/skl: Propagate the error if we fail to find a suitable DPLL divider
 drm/i915/skl: Display the WRPLL frequency we couldn't accomodate when failing
 drm/i915/skl: Make sure to break when not finding suitable PLL dividers

Hardware:
00:02.0 VGA compatible controller [0300]: Intel Corporation Device
[8086:1916] (rev 07) (prog-if 00 [VGA controller])
Subsystem: ASUSTeK Computer Inc. Device [1043:1c3d]
Flags: bus master, fast devsel, latency 0, IRQ 16
Memory at de000000 (64-bit, non-prefetchable) [size=16M]
Memory at c0000000 (64-bit, prefetchable) [size=256M]
I/O ports at f000 [size=64]
Expansion ROM at <unassigned> [disabled]
Capabilities: [40] Vendor Specific Information: Len=0c <?>
Capabilities: [70] Express Root Complex Integrated Endpoint, MSI 00
Capabilities: [ac] MSI: Enable- Count=1/1 Maskable- 64bit-
Capabilities: [d0] Power Management version 2
Capabilities: [100] #1b
Capabilities: [200] Address Translation Service (ATS)
Capabilities: [300] #13
Kernel driver in use: i915

Thanks
Daniel