[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 #33 from Tvrtko Ursulin <tvrtko.ursulin at linux.intel.com> ---
It seems that the number of DC state transitions done by the DMC (0x80030) is
correlated to the activity on the GT IIR. 

Initially I was trying to correlate with the number of GT interrupts, which had
a correlation, but also strangely the number of DC transitions could be higher
than the number of interrupts (up to exactly double!). But if we consider that
one GT interrupt can have multiple GT IIR accesses (one write from GT, one
write from the CPU to clear it), then it starts making more sense.

And in fact nosing around the DMC fw I can spot MMIO addresses of the GT IIR
registers (among other things).

So the question I think is - why is DMC fw looking at GT IIR registers, and
even if it has to for some reason, why it is triggering DC state transitions in
case of pure GT command submission with no display activity whatsoever?

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