[Bug 54226] [snb 3.5] stale semaphore sync seqno (typically as seen on bcs->rcs)

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=54226

--- Comment #28 from Chris Wilson <chris at chris-wilson.co.uk> ---
(In reply to comment #27)
> (In reply to comment #24)
> > Mikhail, for the time being you can set i915.semaphores=0 (or echo 0 >
> > /sys/modules/i915/parameters/semaphores) to prevent this hang.
> 
> What are the consequences?

Rendering throughput is dropped by 10% with SNA, or as much as 3x with UXA.
OpenGL performance is likely to be reduced by about 30%. More CPU time is spent
waiting for the GPU with rc6 disabled, so increased power consumption.

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