[Intel-gfx] [PATCH 72/89] drm/i915/skl: Enable/disable power well for aux transaction

[Daniel Vetter]

On Mon, Nov 10, 2014 at 09:21:47PM +0200, Imre Deak wrote:
> On Fri, 2014-11-07 at 12:08 +0000, Damien Lespiau wrote:
> > On Tue, Sep 16, 2014 at 04:19:07PM +0300, Imre Deak wrote:
> > > > @@ -1233,6 +1233,9 @@ static bool edp_panel_vdd_on(struct intel_dp *intel_dp)
> > > >  	power_domain = intel_display_port_power_domain(intel_encoder);
> > > >  	intel_display_power_get(dev_priv, power_domain);
> > > >  
> > > > +	power_domain = intel_display_aux_power_domain(intel_encoder);
> > > > +	intel_display_power_get(dev_priv, power_domain);
> > > > +
> > > 
> > > The AUX power domains were added to save power when only AUX
> > > functionality is needed, since then we don't need to power on the power
> > > domain needed for full port functionality.
> > 
> > Hum I'm not sure about this. It seems to me that the value of the AUX
> > power domain is to be able to shutdown the AUX hardware when AUX is not
> > needed. That's slightly different from what you're saying;
> 
> Ok, I didn't describe all uses cases where the AUX domains are useful,
> your description here was more complete. To summarize that for later
> reference:
> 
> 1. only AUX (output inactive, we only do a connector detection)
> 2. only main lanes (most of the time when the output is active)
> 3. AUX+main lanes (link training/re-training)
> 4. no AUX, no main lanes (output is inactive)
> 
> > The cases where "only AUX functionality is needed" seem very transient
> > to me, while having the main lanes working and no need for AUX sounds
> > like the case where it's interesting to have the AUX hw powered down.
> > Of course, with PSR we can do both.
> 
> Perhaps, if case 1. above isn't very frequent. But my arguments were
> valid even for case 2. and 3.

I've thought the point of case 1 is that we don't have to fire up the
entire display clocks and power wells when just doing a few dp aux
transactions to probe for outputs.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch