[Mesa-dev] [PATCH 10/10] i965: Require softpin support for Cannonlake and later.

[Chris Wilson]

Quoting Daniel Vetter (2018-05-04 09:45:15)
> On Fri, May 04, 2018 at 01:16:29AM -0700, Kenneth Graunke wrote:
> > On Friday, May 4, 2018 12:39:12 AM PDT Chris Wilson wrote:
> > > Quoting Kenneth Graunke (2018-05-04 08:34:07)
> > > > On Thursday, May 3, 2018 11:53:24 PM PDT Chris Wilson wrote:
> > > > > Quoting Kenneth Graunke (2018-05-04 02:12:40)
> > > > > > This isn't strictly necessary, but anyone running Cannonlake will
> > > > > > already have Kernel 4.5 or later, so there's no reason to support
> > > > > > the relocation model on Gen10+.
> > > > > 
> > > > > /o\ gvt. Need I say more?
> > > > > -Chris
> > > > 
> > > > Yes.  What's the deal with GVT?
> > > 
> > > Their current restrictions involve forcing the use of a 32b
> > > aliasing-ppgtt. Not that they support cnl+ yet, so they might remember
> > > to lift that restriction in time.
> > > -Chris
> > 
> > Wow, that's really miserable.  So, we can't actually depend on real
> > PPGTT existing?  Do you know if/when they might fix this?
> > 
> > This seriously wrecks a lot of my plans if we can't assume PPGTT
> > and have to deal with relocations for all of eternity.  Jason and I
> > have been planning on doing PPGTT-only drivers for months.  We figured
> > that full PPGTT had been working since Gen8 and surely would be viable
> > on anything modern.  If it weren't for enterprise kernels, I would have
> > required this all the way back to Gen8 in a heartbeat.
> 
> softpin with aliasing ppgtt should still work, just thrash really badly.
> It only stops working when there's something pinned at a given offset.

Stops working = -ENOSPC returned to userspace (-EINVAL if it was
soft-pinned by the same batch). Having the error there is the batch
conflicts with a scanout, makes aliasing-ppgtt unusable for userspace if
you haven't been tracking the relocations for this batch, and so can't
redo those post-hoc as you would then likely have to drop the batch on
the floor.
-Chris