[Intel-gfx] [PATCH 2/4] drm/i915: Disable/enable planes as the first/last thing during modeset on gmch platforms

[Ville Syrjälä]

On Fri, May 09, 2014 at 07:22:16AM +0100, Chris Wilson wrote:
> On Thu, May 08, 2014 at 07:23:14PM +0300, ville.syrjala at linux.intel.com wrote:
> > From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> > 
> > We already moved the plane disable/enable to happen as the first/last
> > thing on every other platforms. Follow suit with gmch platforms.
> 
> There is merit in the argument here. Still nerve wracking. Will need to
> check to see if there a counter-argument in the specs.

I've gone through the specs and didn't spot anything explicit. Some
of the modeset sequences do list enabling planes before the ports
and disabling planes after the ports though. But eg. in gen2/3 specs
there are two different sequences: one for modeset, and one for
enable/disable. In the modeset sequence only planes and pipes are
toggled off and on and ports are left enabled for the whole time,
and in the enable/disable sequence ports are turned off first and
back on last. So I think in general you can do these in any order
you like as long as planes aren't enabled when the pipe isn't.

Also we can turn the planes on and off as we like while everything is
up and running so I don't see why the modeset sequence should really
care about planes.

And we should really be able to light up the display without any planes
being active, eg. if a screensaver just turns off all the planes to
show a black screen and then DPMS kicks in and turns everything off.
When it comes back from DPMS the user should still see the same black
screen until some plane gets enabled.

But still if you find anything alarming in the specs I may need to
rethink this.

-- 
Ville Syrjälä
Intel OTC