dm: clk: Define clk_get_parent() for clk operations

This commit adds the clk_get_parent() function, which is responsible
for getting the parent's struct clock pointer.

U-Boot's DM support for getting parent is different (the parent
relationship is in udevice) than the one in Common Clock Framework [CCF]
in Linux. To obtain the pointer to struct clk of parent the
pdev->uclass_priv field is read via dev_get_clk_ptr() wrapper.

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Peng Fan <peng.fan@nxp.com>

drivers/clk/clk-uclass.c

@@ -13,6 +13,7 @@
 #include <dm/read.h>
 #include <dt-structs.h>
 #include <errno.h>
+#include <linux/clk-provider.h>
 
 static inline const struct clk_ops *clk_dev_ops(struct udevice *dev)
 {
@@ -379,6 +380,21 @@ ulong clk_get_rate(struct clk *clk)
 	return ops->get_rate(clk);
 }
 
+struct clk *clk_get_parent(struct clk *clk)
+{
+	struct udevice *pdev;
+	struct clk *pclk;
+
+	debug("%s(clk=%p)\n", __func__, clk);
+
+	pdev = dev_get_parent(clk->dev);
+	pclk = dev_get_clk_ptr(pdev);
+	if (!pclk)
+		return ERR_PTR(-ENODEV);
+
+	return pclk;
+}
+
 ulong clk_set_rate(struct clk *clk, ulong rate)
 {
 	const struct clk_ops *ops = clk_dev_ops(clk->dev);

include/clk.h

@@ -258,6 +258,15 @@ int clk_free(struct clk *clk);
  */
 ulong clk_get_rate(struct clk *clk);
 
+/**
+ * clk_get_parent() - Get current clock's parent.
+ *
+ * @clk:	A clock struct that was previously successfully requested by
+ *		clk_request/get_by_*().
+ * @return pointer to parent's struct clk, or error code passed as pointer
+ */
+struct clk *clk_get_parent(struct clk *clk);
+
 /**
  * clk_set_rate() - Set current clock rate.
  *