drivers/of/base.c: Add of_property_read_u64_index

There is of_property_read_u32_index but no u64 variant. This patch
adds one similar to the u32 version for u64.
Signed-off-by: Alistair Popple <alistair@popple.id.au>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

drivers/of/base.c

@@ -1212,6 +1212,37 @@ int of_property_read_u32_index(const struct device_node *np,
 }
 EXPORT_SYMBOL_GPL(of_property_read_u32_index);
 
+/**
+ * of_property_read_u64_index - Find and read a u64 from a multi-value property.
+ *
+ * @np:		device node from which the property value is to be read.
+ * @propname:	name of the property to be searched.
+ * @index:	index of the u64 in the list of values
+ * @out_value:	pointer to return value, modified only if no error.
+ *
+ * Search for a property in a device node and read nth 64-bit value from
+ * it. Returns 0 on success, -EINVAL if the property does not exist,
+ * -ENODATA if property does not have a value, and -EOVERFLOW if the
+ * property data isn't large enough.
+ *
+ * The out_value is modified only if a valid u64 value can be decoded.
+ */
+int of_property_read_u64_index(const struct device_node *np,
+				       const char *propname,
+				       u32 index, u64 *out_value)
+{
+	const u64 *val = of_find_property_value_of_size(np, propname,
+					((index + 1) * sizeof(*out_value)),
+					0, NULL);
+
+	if (IS_ERR(val))
+		return PTR_ERR(val);
+
+	*out_value = be64_to_cpup(((__be64 *)val) + index);
+	return 0;
+}
+EXPORT_SYMBOL_GPL(of_property_read_u64_index);
+
 /**
  * of_property_read_variable_u8_array - Find and read an array of u8 from a
  * property, with bounds on the minimum and maximum array size.

include/linux/of.h

@@ -292,6 +292,9 @@ extern int of_property_count_elems_of_size(const struct device_node *np,
 extern int of_property_read_u32_index(const struct device_node *np,
 				       const char *propname,
 				       u32 index, u32 *out_value);
+extern int of_property_read_u64_index(const struct device_node *np,
+				       const char *propname,
+				       u32 index, u64 *out_value);
 extern int of_property_read_variable_u8_array(const struct device_node *np,
 					const char *propname, u8 *out_values,
 					size_t sz_min, size_t sz_max);