Re: [PATCH] drm/i915: Don't check power domains state in intel_power_domains_init_hw()

[Imre Deak <imre.deak@intel.com>]

On Tue, Aug 28, 2018 at 02:52:20PM +0300, Imre Deak wrote:
> On Tue, Aug 28, 2018 at 12:45:31PM +0100, Chris Wilson wrote:
> > Quoting Imre Deak (2018-08-28 12:40:43)
> > > During power domains initialization we acquire power well references for
> > > power wells in the INIT power domain. The rest of power wells - which
> > > BIOS could have left enabled - we can only acquire references as needed
> > > during display HW readout. Thus during initialization these latter power
> > > wells can have a refcount of 0 while still being enabled. To avoid the
> > > false-positive state mismatch error this causes remove the check from
> > > intel_power_domains_init_hw() and rely on the state check in
> > > intel_power_domains_enable() which follows the HW readout.
> > 
> > Missing from above is a quick explanation of how those extraneous
> > powerwells are sanitizied. If we don't do the HW readout
> > (i915.disable_display?) do we not then leave the powerwell active and so
> > complain in a later verify_state()?
> 
> These power wells (AUX and DDI on ICL) can only be enabled/disabled at
> a specific spot in the modeset sequence, otherwise the power well
> enable / disable operation will time out. That's the reason they're not
> part of the INIT domain. For these we will acquire the references during
> HW readout when noticing that the corresponding display HW block is active
> and drop them when disabling these HW blocks (normally or as part of
> display state sanitizatoionin). That way we'll ensure the proper spot
> mentioned above for power well enabling/disabling.

And that's also the reason why display HW readout had to be added back
for i915.disable_display .

--Imre
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