drm/sun4i: Implement endpoint parsing using kfifo

The commit da82b8785eeb ("drm/sun4i: add components in breadth first
traversal order") implemented a breadth first traversal of our device tree
nodes graph. However, it was relying on the kernel linked lists, and those
are not really safe for addition.

Indeed, in a single pipeline stage, your first stage (ie, the mixer or
fronted) will be queued, and it will be the final iteration of that list as
far as list_for_each_entry_safe is concerned. Then, during that final
iteration, we'll queue another element (the TCON or the backend) that
list_for_each_entry_safe will not account for, and we will leave the loop
without having iterated over all the elements. And since we won't have
built our components list properly, the DRM driver will be left
non-functional.

We can instead use a kfifo to queue and enqueue components in-order, as was
the original intention. This also has the benefit of removing any dynamic
allocation, making the error handling path simpler too. The only thing
we're losing is the ability to tell whether an element has already been
queued, but that was only needed to remove spurious logs, and therefore
purely cosmetic.

This means that this commit effectively reverses e8afb7b67fba ("drm/sun4i:
don't add components that are already in the queue").

Fixes: da82b8785eeb ("drm/sun4i: add components in breadth first traversal order")
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/4ecb323e787918208f6a5d9f0ebba12c62583c98.1508231063.git-series.maxime.ripard@free-electrons.com

drivers/gpu/drm/sun4i/sun4i_drv.c

@@ -11,6 +11,7 @@
  */
 
 #include <linux/component.h>
+#include <linux/kfifo.h>
 #include <linux/of_graph.h>
 #include <linux/of_reserved_mem.h>
 
@@ -222,29 +223,15 @@ static int compare_of(struct device *dev, void *data)
  * matching system handles this for us.
  */
 struct endpoint_list {
-	struct device_node *node;
-	struct list_head list;
+	DECLARE_KFIFO(fifo, struct device_node *, 16);
 };
 
-static bool node_is_in_list(struct list_head *endpoints,
-			    struct device_node *node)
-{
-	struct endpoint_list *endpoint;
-
-	list_for_each_entry(endpoint, endpoints, list)
-		if (endpoint->node == node)
-			return true;
-
-	return false;
-}
-
 static int sun4i_drv_add_endpoints(struct device *dev,
-				   struct list_head *endpoints,
+				   struct endpoint_list *list,
 				   struct component_match **match,
 				   struct device_node *node)
 {
 	struct device_node *port, *ep, *remote;
-	struct endpoint_list *endpoint;
 	int count = 0;
 
 	/*
@@ -304,19 +291,7 @@ static int sun4i_drv_add_endpoints(struct device *dev,
 			}
 		}
 
-		/* skip downstream node if it is already in the queue */
-		if (node_is_in_list(endpoints, remote))
-			continue;
-
-		/* Add downstream nodes to the queue */
-		endpoint = kzalloc(sizeof(*endpoint), GFP_KERNEL);
-		if (!endpoint) {
-			of_node_put(remote);
-			return -ENOMEM;
-		}
-
-		endpoint->node = remote;
-		list_add_tail(&endpoint->list, endpoints);
+		kfifo_put(&list->fifo, remote);
 	}
 
 	return count;
@@ -325,10 +300,11 @@ static int sun4i_drv_add_endpoints(struct device *dev,
 static int sun4i_drv_probe(struct platform_device *pdev)
 {
 	struct component_match *match = NULL;
-	struct device_node *np = pdev->dev.of_node;
-	struct endpoint_list *endpoint, *endpoint_temp;
+	struct device_node *np = pdev->dev.of_node, *endpoint;
+	struct endpoint_list list;
 	int i, ret, count = 0;
-	LIST_HEAD(endpoints);
+
+	INIT_KFIFO(list.fifo);
 
 	for (i = 0;; i++) {
 		struct device_node *pipeline = of_parse_phandle(np,
@@ -337,31 +313,19 @@ static int sun4i_drv_probe(struct platform_device *pdev)
 		if (!pipeline)
 			break;
 
-		endpoint = kzalloc(sizeof(*endpoint), GFP_KERNEL);
-		if (!endpoint) {
-			ret = -ENOMEM;
-			goto err_free_endpoints;
-		}
-
-		endpoint->node = pipeline;
-		list_add_tail(&endpoint->list, &endpoints);
+		kfifo_put(&list.fifo, pipeline);
 	}
 
-	list_for_each_entry_safe(endpoint, endpoint_temp, &endpoints, list) {
+	while (kfifo_get(&list.fifo, &endpoint)) {
 		/* process this endpoint */
-		ret = sun4i_drv_add_endpoints(&pdev->dev, &endpoints, &match,
-					      endpoint->node);
+		ret = sun4i_drv_add_endpoints(&pdev->dev, &list, &match,
+					      endpoint);
 
 		/* sun4i_drv_add_endpoints can fail to allocate memory */
 		if (ret < 0)
-			goto err_free_endpoints;
+			return ret;
 
 		count += ret;
-
-		/* delete and cleanup the current entry */
-		list_del(&endpoint->list);
-		of_node_put(endpoint->node);
-		kfree(endpoint);
 	}
 
 	if (count)
@@ -370,15 +334,6 @@ static int sun4i_drv_probe(struct platform_device *pdev)
 						       match);
 	else
 		return 0;
-
-err_free_endpoints:
-	list_for_each_entry_safe(endpoint, endpoint_temp, &endpoints, list) {
-		list_del(&endpoint->list);
-		of_node_put(endpoint->node);
-		kfree(endpoint);
-	}
-
-	return ret;
 }
 
 static int sun4i_drv_remove(struct platform_device *pdev)