[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 #7 from Imre Deak <imre.deak at intel.com> ---
(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?

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