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

[Chris Wilson]

Quoting Imre Deak (2018-08-28 12:56:38)
> 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 .

Just add the snippet to the changelog: "...as needed during display HW
readout and so must defer sanitization until then (also implying that we
must always do HW readout to cleanup unused powerwells)."

A reminder in code as well:
/* powerwells are sanitized by i_forget_the_function_name() */
-Chris