clk: add devm_get_clk_from_child() API

Some driver is using this type of DT bindings for clock (more detail,
see ${LINUX}/Documentation/devicetree/bindings/sound/simple-card.txt).

sound_soc {
...
cpu {
clocks = <&xxx>;
...
};
codec {
clocks = <&xxx>;
...
};
};

Current driver in this case uses of_clk_get() for each node, but there
is no devm_of_clk_get() today.
OTOH, the problem of having devm_of_clk_get() is that it encourages the
use of of_clk_get() when clk_get() is more desirable.

Thus, this patch adds new devm_get_clk_from_chile() which explicitly
reads as get a clock from a child node of this device.
By this function, we can also use this type of DT bindings

sound_soc {
clocks = <&xxx>, <&xxx>;
clock-names = "cpu", "codec";
clock-ranges;
...
cpu {
...
};
codec {
...
};
};

Signed-off-by: Kuninori Morimoto <[e-mail]>
[[e-mail]: Rename subject to clk + add API]
Signed-off-by: Stephen Boyd <[e-mail]>

drivers/clk/clk-devres.c

···
 	WARN_ON(ret);
 }
 EXPORT_SYMBOL(devm_clk_put);
+
+struct clk *devm_get_clk_from_child(struct device *dev,
+				    struct device_node *np, const char *con_id)
+{
+	struct clk **ptr, *clk;
+
+	ptr = devres_alloc(devm_clk_release, sizeof(*ptr), GFP_KERNEL);
+	if (!ptr)
+		return ERR_PTR(-ENOMEM);
+
+	clk = of_clk_get_by_name(np, con_id);
+	if (!IS_ERR(clk)) {
+		*ptr = clk;
+		devres_add(dev, ptr);
+	} else {
+		devres_free(ptr);
+	}
+
+	return clk;
+}
+EXPORT_SYMBOL(devm_get_clk_from_child);

include/linux/clk.h

···
 #include <linux/notifier.h>
 
 struct device;
-
 struct clk;
+struct device_node;
+struct of_phandle_args;
 
 /**
  * DOC: clk notifier callback types
···
 struct clk *devm_clk_get(struct device *dev, const char *id);
 
 /**
+ * devm_get_clk_from_child - lookup and obtain a managed reference to a
+ *			     clock producer from child node.
+ * @dev: device for clock "consumer"
+ * @np: pointer to clock consumer node
+ * @con_id: clock consumer ID
+ *
+ * This function parses the clocks, and uses them to look up the
+ * struct clk from the registered list of clock providers by using
+ * @np and @con_id
+ *
+ * The clock will automatically be freed when the device is unbound
+ * from the bus.
+ */
+struct clk *devm_get_clk_from_child(struct device *dev,
+				    struct device_node *np, const char *con_id);
+
+/**
  * clk_enable - inform the system when the clock source should be running.
  * @clk: clock source
  *
···
 	return NULL;
 }
 
+static inline struct clk *devm_get_clk_from_child(struct device *dev,
+				struct device_node *np, const char *con_id)
+{
+	return NULL;
+}
+
 static inline void clk_put(struct clk *clk) {}
 
 static inline void devm_clk_put(struct device *dev, struct clk *clk) {}
···
 	clk_disable(clk);
 	clk_unprepare(clk);
 }
-
-struct device_node;
-struct of_phandle_args;
 
 #if defined(CONFIG_OF) && defined(CONFIG_COMMON_CLK)
 struct clk *of_clk_get(struct device_node *np, int index);