drm/i915: Remove livelock from "do_idle_maps" vtd w/a

A call to wait for the GT to idle from inside the put_pages fallback is
prone to cause an uninterruptible livelock. As it does not provide
adequate serialisation with new requests, simply fallback to a trivial
sleep.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201209164008.5487-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -55,22 +55,17 @@ int i915_gem_gtt_prepare_pages(struct drm_i915_gem_object *obj,
 void i915_gem_gtt_finish_pages(struct drm_i915_gem_object *obj,
 			       struct sg_table *pages)
 {
-	struct drm_i915_private *dev_priv = to_i915(obj->base.dev);
-	struct device *kdev = &dev_priv->drm.pdev->dev;
-	struct i915_ggtt *ggtt = &dev_priv->ggtt;
-
-	if (unlikely(ggtt->do_idle_maps)) {
-		/* XXX This does not prevent more requests being submitted! */
-		if (intel_gt_retire_requests_timeout(ggtt->vm.gt,
-						     -MAX_SCHEDULE_TIMEOUT)) {
-			drm_err(&dev_priv->drm,
-				"Failed to wait for idle; VT'd may hang.\n");
-			/* Wait a bit, in hopes it avoids the hang */
-			udelay(10);
-		}
-	}
+	struct drm_i915_private *i915 = to_i915(obj->base.dev);
+	struct i915_ggtt *ggtt = &i915->ggtt;
+
+	/* XXX This does not prevent more requests being submitted! */
+	if (unlikely(ggtt->do_idle_maps))
+		/* Wait a bit, in the hope it avoids the hang */
+		usleep_range(100, 250);
 
-	dma_unmap_sg(kdev, pages->sgl, pages->nents, PCI_DMA_BIDIRECTIONAL);
+	dma_unmap_sg(&i915->drm.pdev->dev,
+		     pages->sgl, pages->nents,
+		     PCI_DMA_BIDIRECTIONAL);
 }
 
 /**