clk: Document of_parse_clkspec() some more

The return value of of_parse_clkspec() is peculiar. If the function is
called with a NULL argument for 'name' it will return -ENOENT, but if
it's called with a non-NULL argument for 'name' it will return -EINVAL.
This peculiarity is documented by commit 5c56dfe6 ("clk: Add comment
about __of_clk_get_by_name() error values").

Let's further document this function so that it's clear what the return
value is and how to use the arguments to parse clk specifiers.

Cc: Phil Edworthy <phil.edworthy@renesas.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>
Link: https://lkml.kernel.org/r/20190826212042.48642-1-sboyd@kernel.orgReviewed-by: Phil Edworthy <phil.edworthy@renesas.com>

drivers/clk/clk.c

@@ -4316,12 +4316,43 @@ void devm_of_clk_del_provider(struct device *dev)
 }
 EXPORT_SYMBOL(devm_of_clk_del_provider);
 
-/*
- * Beware the return values when np is valid, but no clock provider is found.
- * If name == NULL, the function returns -ENOENT.
- * If name != NULL, the function returns -EINVAL. This is because
- * of_parse_phandle_with_args() is called even if of_property_match_string()
- * returns an error.
+/**
+ * of_parse_clkspec() - Parse a DT clock specifier for a given device node
+ * @np: device node to parse clock specifier from
+ * @index: index of phandle to parse clock out of. If index < 0, @name is used
+ * @name: clock name to find and parse. If name is NULL, the index is used
+ * @out_args: Result of parsing the clock specifier
+ *
+ * Parses a device node's "clocks" and "clock-names" properties to find the
+ * phandle and cells for the index or name that is desired. The resulting clock
+ * specifier is placed into @out_args, or an errno is returned when there's a
+ * parsing error. The @index argument is ignored if @name is non-NULL.
+ *
+ * Example:
+ *
+ * phandle1: clock-controller@1 {
+ *	#clock-cells = <2>;
+ * }
+ *
+ * phandle2: clock-controller@2 {
+ *	#clock-cells = <1>;
+ * }
+ *
+ * clock-consumer@3 {
+ *	clocks = <&phandle1 1 2 &phandle2 3>;
+ *	clock-names = "name1", "name2";
+ * }
+ *
+ * To get a device_node for `clock-controller@2' node you may call this
+ * function a few different ways:
+ *
+ *   of_parse_clkspec(clock-consumer@3, -1, "name2", &args);
+ *   of_parse_clkspec(clock-consumer@3, 1, NULL, &args);
+ *   of_parse_clkspec(clock-consumer@3, 1, "name2", &args);
+ *
+ * Return: 0 upon successfully parsing the clock specifier. Otherwise, -ENOENT
+ * if @name is NULL or -EINVAL if @name is non-NULL and it can't be found in
+ * the "clock-names" property of @np.
  */
 static int of_parse_clkspec(const struct device_node *np, int index,
 			    const char *name, struct of_phandle_args *out_args)