drm/i915: Unwind failure on pinning the gen7 ppgtt

If we fail to pin the ggtt vma slot for the ppgtt page tables, we need
to unwind the locals before reporting the error. Or else on subsequent
attempts to bind the page tables into the ggtt, we will already believe
that the vma has been pinned and continue on blithely. If something else
should happen to be at that location, choas ensues.

Fixes: a2bbf71 ("drm/i915/gtt: Only keep gen6 page directories pinned while active")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Cc: <stable@vger.kernel.org> # v4.19+
Reviewed-by: Matthew Auld <matthew.william.auld@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181222030623.21710-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -2073,6 +2073,7 @@ static struct i915_vma *pd_vma_create(struct gen6_hw_ppgtt *ppgtt, int size)
 int gen6_ppgtt_pin(struct i915_hw_ppgtt *base)
 {
 	struct gen6_hw_ppgtt *ppgtt = to_gen6_ppgtt(base);
+	int err;
 
 	/*
 	 * Workaround the limited maximum vma->pin_count and the aliasing_ppgtt
@@ -2088,9 +2089,17 @@ int gen6_ppgtt_pin(struct i915_hw_ppgtt *base)
 	 * allocator works in address space sizes, so it's multiplied by page
 	 * size. We allocate at the top of the GTT to avoid fragmentation.
 	 */
-	return i915_vma_pin(ppgtt->vma,
-			    0, GEN6_PD_ALIGN,
-			    PIN_GLOBAL | PIN_HIGH);
+	err = i915_vma_pin(ppgtt->vma,
+			   0, GEN6_PD_ALIGN,
+			   PIN_GLOBAL | PIN_HIGH);
+	if (err)
+		goto unpin;
+
+	return 0;
+
+unpin:
+	ppgtt->pin_count = 0;
+	return err;
 }
 
 void gen6_ppgtt_unpin(struct i915_hw_ppgtt *base)