clk: respect the clock dependencies in of_clk_init

Until now the clock providers were initialized in the order found in
the device tree. This led to have the dependencies between the clocks
not respected: children clocks could be initialized before their
parent clocks.

Instead of forcing each platform to manage its own initialization order,
this patch adds this work inside the framework itself.

Using the data of the device tree the of_clk_init function now delayed
the initialization of a clock provider if its parent provider was not
ready yet.

The strict dependency check (all parents of a given clk must be
initialized) was added by Boris BREZILLON

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Boris BREZILLON <b.brezillon@overkiz.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>

drivers/clk/clk.c

@@ -2539,24 +2539,99 @@ const char *of_clk_get_parent_name(struct device_node *np, int index)
 }
 EXPORT_SYMBOL_GPL(of_clk_get_parent_name);
 
+struct clock_provider {
+	of_clk_init_cb_t clk_init_cb;
+	struct device_node *np;
+	struct list_head node;
+};
+
+static LIST_HEAD(clk_provider_list);
+
+/*
+ * This function looks for a parent clock. If there is one, then it
+ * checks that the provider for this parent clock was initialized, in
+ * this case the parent clock will be ready.
+ */
+static int parent_ready(struct device_node *np)
+{
+	int i = 0;
+
+	while (true) {
+		struct clk *clk = of_clk_get(np, i);
+
+		/* this parent is ready we can check the next one */
+		if (!IS_ERR(clk)) {
+			clk_put(clk);
+			i++;
+			continue;
+		}
+
+		/* at least one parent is not ready, we exit now */
+		if (PTR_ERR(clk) == -EPROBE_DEFER)
+			return 0;
+
+		/*
+		 * Here we make assumption that the device tree is
+		 * written correctly. So an error means that there is
+		 * no more parent. As we didn't exit yet, then the
+		 * previous parent are ready. If there is no clock
+		 * parent, no need to wait for them, then we can
+		 * consider their absence as being ready
+		 */
+		return 1;
+	}
+}
+
 /**
  * of_clk_init() - Scan and init clock providers from the DT
  * @matches: array of compatible values and init functions for providers.
  *
- * This function scans the device tree for matching clock providers and
- * calls their initialization functions
+ * This function scans the device tree for matching clock providers
+ * and calls their initialization functions. It also do it by trying
+ * to follow the dependencies.
  */
 void __init of_clk_init(const struct of_device_id *matches)
 {
 	const struct of_device_id *match;
 	struct device_node *np;
+	struct clock_provider *clk_provider, *next;
+	bool is_init_done;
+	bool force = false;
 
 	if (!matches)
 		matches = &__clk_of_table;
 
+	/* First prepare the list of the clocks providers */
 	for_each_matching_node_and_match(np, matches, &match) {
-		of_clk_init_cb_t clk_init_cb = match->data;
-		clk_init_cb(np);
+		struct clock_provider *parent =
+			kzalloc(sizeof(struct clock_provider),	GFP_KERNEL);
+
+		parent->clk_init_cb = match->data;
+		parent->np = np;
+		list_add(&parent->node, &clk_provider_list);
+	}
+
+	while (!list_empty(&clk_provider_list)) {
+		is_init_done = false;
+		list_for_each_entry_safe(clk_provider, next,
+					&clk_provider_list, node) {
+			if (force || parent_ready(clk_provider->np)) {
+				clk_provider->clk_init_cb(clk_provider->np);
+				list_del(&clk_provider->node);
+				kfree(clk_provider);
+				is_init_done = true;
+			}
+		}
+
+		/*
+		 * We didn't managed to initialize any of the
+		 * remaining providers during the last loop, so now we
+		 * initialize all the remaining ones unconditionally
+		 * in case the clock parent was not mandatory
+		 */
+		if (!is_init_done)
+			force = true;
+
 	}
 }
 #endif