[Intel-gfx] [PATCH 1/2] drm/i915: Disallow pin ioctl completely for kms drivers

[Daniel Vetter]

On Mon, Nov 24, 2014 at 10:35:29AM +0000, Chris Wilson wrote:
> On Mon, Nov 24, 2014 at 11:30:24AM +0100, Daniel Vetter wrote:
> > The problem here is that SNA pins batchbuffers to etch out a bit more
> > performance. Iirc it started out as a w/a for i830M (which we've
> > implemented in the kernel since a long time already).
> 
> Hmm, we only finally fixed it in the kernel a couple of months ago.

The original wa hack has landed in 3.8:

commit b45305fce5bb1abec263fcff9d81ebecd6306ede
Author: Daniel Vetter <daniel.vetter at ffwll.ch>
Date:   Mon Dec 17 16:21:27 2012 +0100

    drm/i915: Implement workaround for broken CS tlb on i830/845

The follow-up fix from you was cc: stable:

commit c4d69da167fa967749aeb70bc0e94a457e5d00c1
Author: Chris Wilson <chris at chris-wilson.co.uk>
Date:   Mon Sep 8 14:25:41 2014 +0100

    drm/i915: Evict CS TLBs between batches

So as long as people backport all of this and don't try to run it on 3.8
we should be fine I think wrt backwards/forwards compat. I'll add this to
the commit message.

> > The problem is
> > that the pin ioctl wasn't added in
> > 
> > commit d23db88c3ab233daed18709e3a24d6c95344117f
> > Author: Chris Wilson <chris at chris-wilson.co.uk>
> > Date:   Fri May 23 08:48:08 2014 +0200
> > 
> >     drm/i915: Prevent negative relocation deltas from wrapping
> > 
> > Fix this by simply disallowing pinning from userspace so that the
> > kernel is in full control of batch placement again. Especially since
> > distros are moving towards running X as non-root, so most users won't
> > even be able to see any benefits.
> 
> Pinning is a useful tool and it would also be useful to have again on
> gen6+.

I think softpin or similar is doable with ppgtt. But with shared ggtt I'm
not really enthusiastic about providing this kind of rope to userspace.
And softpin is a different type of pinning, so we don't really lose
anything by ripping out the userspace hard pinning ioctls.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch