[Bug 100572] [SKL dmc] Headless mode media transcoding is 20-30% slower comparing to connected monitor use case

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=100572

--- Comment #8 from Imre Deak <imre.deak at intel.com> ---
(In reply to Imre Deak from comment #7)
> (In reply to Tvrtko Ursulin from comment #6)
> > I tried not loading the DMC firmware and can confirm that the issue is not
> > present in that case.
> > 
> > Also, it is possible to reproduce this in the default kernel config (no
> > pinning is required) simply with igt/benchmarks/gem_latency -n 0 in which
> > case the perf difference between the two setups was ~8x in my testing.
> 
> One possibility is that DC6 enables deeper system level power states and
> this causes latency elsewhere. What are the PC state residencies shown by
> powertop or the kernel's tools/power/x86/turbostat when DMC is loaded and
> not?
> 
> What's the effect of limiting max_cstates to 0 (and having DMC loaded)?
> 
> An other problem could be that the GPU is trying to access the display,
> (maybe checking scan line counts or something?).
> 
> Does /sys/kernel/debug/dri/0/i915_dmc_info show any transitions during the
> test when DMC is loaded?

Also the actual RC6 residency reported by powertop/turbostat would be
interesting. (even though it's disabled)

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