[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 #28 from Imre Deak <imre.deak at intel.com> ---
(In reply to Tvrtko Ursulin from comment #27)
> (In reply to Imre Deak from comment #26)
> > (In reply to Tvrtko Ursulin from comment #25)
> > > One of the experiments Animesh asked me to do was to look at the state of
> > > the DC_STATE_EN register at runtime.
> > > 
> > > It looked that it had reverted to a value other than what was programmed by
> > > i915. On top of that, it looked impossible to manually modify it at runtime.
> > > 
> > > The value it is reverting to was DC_STATE_EN_UPTO_DC6, regardless of whether
> > > the driver has programmed DC_STATE_EN_UPTO_DC5 or DC_STATE_EN_UPTO_DC5 |
> > > DC_STATE_EN_UPTO_DC6.
> > > 
> > > This coupled with a comment in gen9_write_dc_state makes me suspicious
> > > whether DMC is not doing things behind the drivers back?
> > 
> > We don't use DC5 on SKL normally only DC6. Did you boot with i915.enable_dc
> > set to something non-default?
> 
> Not via the param but on Animesh'es suggestion I had the gen9_write_dc_state
> modified to only ever program DC_STATE_EN_UPTO_DC5, if non zero state was
> passed in. Even after that the read back from DC_STATE_EN at runtime was
> DC_STATE_EN_UPTO_DC6. But during programming the read-back was getting the
> programmed value. So I guess some time after the initial programming it gets
> modified by someone. And I couldn't find any other place in i915 which would
> do it. Which is why I thought it could be DMC.

Yes, as I understand there is a firmware bug that prevents using DC5 on SKL.
After exiting a low-power DC state it will always "restore"
DC_STATE_EN_UPTO_DC6 to DC_STATE_EN regardless of what was programmed there.

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