drm/i915/gt: Prefer recycling an idle fence

If we want to reuse a fence that is in active use by the GPU, we have to
wait an uncertain amount of time, but if we reuse an inactive fence, we
can change it right away. Loop through the list of available fences
twice, ignoring any active fences on the first pass.
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/20201223122051.4624-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gt/intel_ggtt_fencing.c

@@ -320,13 +320,31 @@ void i915_vma_revoke_fence(struct i915_vma *vma)
 		fence_write(fence);
 }
 
+static bool fence_is_active(const struct i915_fence_reg *fence)
+{
+	return fence->vma && i915_vma_is_active(fence->vma);
+}
+
 static struct i915_fence_reg *fence_find(struct i915_ggtt *ggtt)
 {
-	struct i915_fence_reg *fence;
+	struct i915_fence_reg *active = NULL;
+	struct i915_fence_reg *fence, *fn;
 
-	list_for_each_entry(fence, &ggtt->fence_list, link) {
+	list_for_each_entry_safe(fence, fn, &ggtt->fence_list, link) {
 		GEM_BUG_ON(fence->vma && fence->vma->fence != fence);
 
+		if (fence == active) /* now seen this fence twice */
+			active = ERR_PTR(-EAGAIN);
+
+		/* Prefer idle fences so we do not have to wait on the GPU */
+		if (active != ERR_PTR(-EAGAIN) && fence_is_active(fence)) {
+			if (!active)
+				active = fence;
+
+			list_move_tail(&fence->link, &ggtt->fence_list);
+			continue;
+		}
+
 		if (atomic_read(&fence->pin_count))
 			continue;