[Intel-gfx] [PATCH 27/29] drm/i915: Add save/restore of SWF for ILK+

[Ville Syrjälä]

On Fri, Nov 06, 2015 at 01:14:33PM +0000, Chris Wilson wrote:
> On Wed, Nov 04, 2015 at 11:20:15PM +0200, ville.syrjala at linux.intel.com wrote:
> > From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> > 
> > No idea if we might want these. Perhaps there is a "keep your paws off
> > my GPU" bit in there somewhere to avoid BIOS crap?
> 
> Hmm. "These registers are used as scratch pad data storage space and
> have no direct effect on hardware operation.  The use of these
> registers is defined by the software architecture."
> 
> Not sure. But I do know that some of that scratch space was repurposed
> for use by the hardware (but the ones I know about don't need to be
> restored).
> 
> Do you have anything more definite than a hunch? If you have a good reason,
> Acked-by: Chris Wilson <chris at chris-wilson.co.uk>
> (as in I checked that those registers do exist as a 36x32bit block)

Basically I have nothing. I have pondered about what the BIOS might be
doing with these, and there are certainly interesting bits in there (eg.
"driver loaded", "driver does display switch instead of bios", "choose
between apm vs. acpi vs. acpi+extension", etc.). So I've been thinking
that maybe we could get rid of the lid notify stuff if we set the right
bit, and/or maybe find something for the D3 vs. hibernate issue as well.
I think there was some bit about GTT restore too.

Another interesting thing I just noticed is that according to some docs
VLV/CHV might have both the GMCH registers, and the ILK+ registers (just
housed in the Gunit on VLV/CHV). No real idea which set of registers
those platforms would actually use.

-- 
Ville Syrjälä
Intel OTC