usb: common: add helper to get role-switch-default-mode

Add helper to get "role-switch-default-mode", and convert it
to the corresponding enum usb_dr_mode

Signed-off-by: Chunfeng Yun <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/common/common.c

···
 EXPORT_SYMBOL_GPL(usb_get_dr_mode);
 
 /**
+ * usb_get_role_switch_default_mode - Get default mode for given device
+ * @dev: Pointer to the given device
+ *
+ * The function gets string from property 'role-switch-default-mode',
+ * and returns the corresponding enum usb_dr_mode.
+ */
+enum usb_dr_mode usb_get_role_switch_default_mode(struct device *dev)
+{
+	const char *str;
+	int ret;
+
+	ret = device_property_read_string(dev, "role-switch-default-mode", &str);
+	if (ret < 0)
+		return USB_DR_MODE_UNKNOWN;
+
+	return usb_get_dr_mode_from_string(str);
+}
+EXPORT_SYMBOL_GPL(usb_get_role_switch_default_mode);
+
+/**
  * usb_decode_interval - Decode bInterval into the time expressed in 1us unit
  * @epd: The descriptor of the endpoint
  * @speed: The speed that the endpoint works as

include/linux/usb/otg.h

···
  * and returns the corresponding enum usb_dr_mode
  */
 extern enum usb_dr_mode usb_get_dr_mode(struct device *dev);
+extern enum usb_dr_mode usb_get_role_switch_default_mode(struct device *dev);
 
 #endif /* __LINUX_USB_OTG_H */