[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 #21 from Tvrtko Ursulin <tvrtko.ursulin at linux.intel.com> ---
(In reply to Sunil Kamath from comment #20)
> Some details and further tests to be tried are:
> 
> Though by avoiding DMC fw load, issue is going away issue is not lilted just
> to DMC fw. (proved by other tests results)

Which results are you referring to? So far I haven't been able to reproduce the
performance regression without the DMC loaded. But yeah, it is possible it is
an driver - firmware interaction of some sorts.

> Real way to address this is to gracefully handle power infra change for a
> headless system.
> 
> “i915.disable_power_wells=0” fixes performance issue on DP too?

It fixes the issue in headless mode and headless mode cannot have DP connected
so not sure what you mean by this?

What Dmitry observed in #19 is that forcing the DP connector on hasn't got the
same workaround effect as forcing the HDMI on does. As far as I could see,
there is some difference in the display code paths, where forcing the DP on
does not actually turn on the power well(s). Contrary to forcing the HDMI
connector to on which does this. There is also a difference in system behaviour
depending on presence or absence of fbcon, but this is just a secondary effect
of modeset happening or not when the connector is forced.

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