drm/i915: Remove walk over obj->vma_list for the shrinker

In the next patch, we want to reduce the lock coverage within the
shrinker, and one of the dangerous walks we have is over obj->vma_list.
We are only walking the obj->vma_list in order to check whether it has
been permanently pinned by HW access, typically via use on the scanout.
But we have a couple of other long term pins, the context objects for
which we currently have to check the individual vma pin_count. If we
instead mark these using obj->pin_display, we can forgo the dangerous
and sometimes slow list iteration.

v2: Rearrange code to try and avoid confusion from false associations
due to arrangement of whitespace along with rebasing on obj->pin_global.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171013202621.7276-4-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -1244,6 +1244,8 @@ int intel_ring_pin(struct intel_ring *ring,
 	if (IS_ERR(addr))
 		goto err;
 
+	vma->obj->pin_global++;
+
 	ring->vaddr = addr;
 	return 0;
 
@@ -1275,6 +1277,7 @@ void intel_ring_unpin(struct intel_ring *ring)
 		i915_gem_object_unpin_map(ring->vma->obj);
 	ring->vaddr = NULL;
 
+	ring->vma->obj->pin_global--;
 	i915_vma_unpin(ring->vma);
 }
 
@@ -1439,6 +1442,7 @@ intel_ring_context_pin(struct intel_engine_cs *engine,
 			goto err;
 
 		ce->state->obj->mm.dirty = true;
+		ce->state->obj->pin_global++;
 	}
 
 	/* The kernel context is only used as a placeholder for flushing the
@@ -1473,8 +1477,10 @@ static void intel_ring_context_unpin(struct intel_engine_cs *engine,
 	if (--ce->pin_count)
 		return;
 
-	if (ce->state)
+	if (ce->state) {
+		ce->state->obj->pin_global--;
 		i915_vma_unpin(ce->state);
+	}
 
 	i915_gem_context_put(ctx);
 }