Re: [PATCH] drm/i915: Add documentation to gen9_set_dc_state()

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

On Wed, Apr 25, 2018 at 11:47:28AM -0700, Dhinakaran Pandiyan wrote:
> On Tue, 2018-04-17 at 14:31 +0300, Imre Deak wrote:
> > Add documentation to gen9_set_dc_state() on what enabling a given DC
> > state means and at what point HW/DMC actually enters/exits these states.
> > 
> > Cc: Jani Nikula <jani.nikula@intel.com>
> > Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> > Signed-off-by: Imre Deak <imre.deak@intel.com>
> > ---
> >  drivers/gpu/drm/i915/intel_runtime_pm.c | 23 +++++++++++++++++++++++
> >  1 file changed, 23 insertions(+)
> > 
> > [ On IRC I stated that PSR entry would be prevented in a given DC state,
> >   but looking more at it I haven't found any proof for this. So as I
> >   understand the only connection between PSR and DC states is that if
> >   DC5/6 is disabled power saving will be blocked, which would otherwise
> >   be possible when PSR is active and the display pipe is off. ]
> > 
> > diff --git a/drivers/gpu/drm/i915/intel_runtime_pm.c b/drivers/gpu/drm/i915/intel_runtime_pm.c
> > index 53ea564f971e..40a7955886d4 100644
> > --- a/drivers/gpu/drm/i915/intel_runtime_pm.c
> > +++ b/drivers/gpu/drm/i915/intel_runtime_pm.c
> > @@ -542,6 +542,29 @@ void gen9_sanitize_dc_state(struct drm_i915_private *dev_priv)
> >  	dev_priv->csr.dc_state = val;
> >  }
> >  
> > +/**
> > + * gen9_set_dc_state - set target display C power state
> > + * @dev_priv: i915 device instance
> > + * @state: target DC power state
> > + * - DC_STATE_DISABLE
> > + * - DC_STATE_EN_UPTO_DC5
> > + * - DC_STATE_EN_UPTO_DC6
> > + * - DC_STATE_EN_DC9
> > + *
> > + * Signal to DMC firmware/HW the target DC power state passed in @state.
> > + * DMC/HW can turn off individual display clocks and power rails when entering
> 
> Any chance you could eliminate the firmware/HW ambiguity? Not knowing
> whether it is the firmware or the HW leads to potentially incorrect
> assumptions.

It's not specified. What I understood about current DMC firmware blobs
is that they will prevent entering DC5/6 if there is a pending
GT/display interrupt. So all other gating - for instance based on a
pipe's active state - happens outside of the firmware.

> > + * a deeper DC power state (higher in number) and turns these back when exiting
> > + * that state to a shallower power state (lower in number). The HW will decide
> > + * when to actually enter a given state on an on-demand basis, for instance
> > + * depending on the active state of display pipes. The state of display
> > + * registers backed by affected power rails are saved/restored as needed.
> > + *
> 
> One of things that is completely misleading is the enable_dc6 debug
> message we print. It is all over dmesg, but like you already wrote, HW
> needs PSR to be enabled and only in a single-pipe active case, the
> display controller can go to DC6. So, with PSR disabled upstream, there
> is really no chance that the HW can go to DC6 without all displays
> disabled.

Yes, that is how things work. The driver only enables - or allows if you
like - these states and this enabling is what the debug message is about
The actual toggling of power rails, clocks etc. will happen behind the
scenes. This is similar to how RC6 works for instance, with DC states
you just need more assistance atm from the driver, hence the need for
more cases where we have to disable/re-enable DC states.

> > + * Based on the above enabling a deeper DC power state is asynchronous wrt.
> > + * enabling it. Disabling a deeper power state is synchronous: for instance
> > + * setting %DC_STATE_DISABLE won't complete until all HW resources are turned
> > + * back on and register state is restored. This is guaranteed by the MMIO write
> > + * to DC_STATE_EN blocking until the state is restored.
> > + */
> >  static void gen9_set_dc_state(struct drm_i915_private *dev_priv, uint32_t state)
> >  {
> >  	uint32_t val;
> 
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