[Intel-gfx] [PATCH] drm/i915: respect previous reg values on primary plane disable

[Ville Syrjälä]

On Wed, Oct 14, 2015 at 11:59:59AM -0700, Kevin Strasser wrote:
> On Wed, Oct 14, 2015 at 03:22:23PM +0300, Ville Syrjälä wrote:
> > On Wed, Oct 14, 2015 at 01:12:27PM +0100, Chris Wilson wrote:
> > > On Wed, Oct 14, 2015 at 03:07:41PM +0300, Ville Syrjälä wrote:
> > > > On Tue, Oct 13, 2015 at 02:24:41PM -0700, Kevin Strasser wrote:
> [...]
> > > > > -		I915_WRITE(reg, 0);
> > > > > +		I915_WRITE(reg, I915_READ(reg) & ~DISPLAY_PLANE_ENABLE);
> > > > 
> > > > Eh, what now? We've been trying to eliminate these nasty RMWs.
> > > > 
> > > > Are you saying that if we disabled the plane, but leave the "pass plane
> > > > data through gamma" it still affects the output for any pixel "covered"
> > > > by the disabled plane?
> > > 
> > > What I thought was being said was that if a plane is set to black (but
> > > with gamma enabled on the pipe) then a different CRC is produced
> > > compared to when the pipe is completely disabled (no plane at all). It
> > > sounded to me like a test case failure.
> > 
> > In that case I don't understand how the patch is supposed to help.
> > 
> > But yeah, tests like these should really set up an identity gamma
> > and pipe csc matrix.
> > 
> > Also we should grow some properties to control whether the plane
> > data passes through the gamma/csc or not. Those could then be used
> > to achieeve the same effect.
> 
> Just to level set, these cases will produce different CRCs on HSW:
>  1. Primary plane disabled, gamma correction disabled
>  2. Primary plane disabled, gamma correction enabled
> 
> Case 2 is visibly brighter than case 1 and looks more like the enabled black
> primary plane case.

Ugh. That's weird. I thought data not going through any plane would
bypass the gamma too.

> The purpose of this patch is to get the behavior of a
> disabled primary plane to match that of an enabled black plane, just as it does
> on non-HSW platforms.

Does it? I just tried it on IVB, and behaves just like you said. So not
sure how far back this goes.

And now I'm really wondering about platforms where the primary
plane need not be fullscreen (gen2/3 and chv).

I tried this on SKL too, but that confused me even more. The data not
going through any plane seems to be gamma corrected regardless of any
plane control bits, so that's good. However the legacy palette seems
all fubar. Black input apparently doesn't map to palette entry 0.
I wonder if you're seeing this on HSW too, or is your palette entry 0
supposed to be non-black?

Looks like quite a bit more testing is needed to get to the bottom of
this.

-- 
Ville Syrjälä
Intel OTC