usb: gadget: core: introduce ->udc_set_speed() method

Sometimes, the gadget driver we want to run has max_speed lower than
what the UDC supports. In such situations, UDC might want to make sure
we don't try to connect on speeds not supported by the gadget
driver (e.g. super-speed capable dwc3 with high-speed capable g_midi)
because that will just fail.

In order to make sure this situation never happens, we introduce a new
optional ->udc_set_speed() method which can be implemented by
interested UDC drivers.
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/gadget/udc/core.c

@@ -1055,6 +1055,23 @@ static inline void usb_gadget_udc_stop(struct usb_udc *udc)
 	udc->gadget->ops->udc_stop(udc->gadget);
 }
 
+/**
+ * usb_gadget_udc_set_speed - tells usb device controller speed supported by
+ *    current driver
+ * @udc: The device we want to set maximum speed
+ * @speed: The maximum speed to allowed to run
+ *
+ * This call is issued by the UDC Class driver before calling
+ * usb_gadget_udc_start() in order to make sure that we don't try to
+ * connect on speeds the gadget driver doesn't support.
+ */
+static inline void usb_gadget_udc_set_speed(struct usb_udc *udc,
+					    enum usb_device_speed speed)
+{
+	if (udc->gadget->ops->udc_set_speed)
+		udc->gadget->ops->udc_set_speed(udc->gadget, speed);
+}
+
 /**
  * usb_udc_release - release the usb_udc struct
  * @dev: the dev member within usb_udc
@@ -1288,6 +1305,9 @@ static int udc_bind_to_driver(struct usb_udc *udc, struct usb_gadget_driver *dri
 	udc->dev.driver = &driver->driver;
 	udc->gadget->dev.driver = &driver->driver;
 
+	if (driver->max_speed < udc->gadget->max_speed)
+		usb_gadget_udc_set_speed(udc, driver->max_speed);
+
 	ret = driver->bind(udc->gadget, driver);
 	if (ret)
 		goto err1;

include/linux/usb/gadget.h

@@ -304,6 +304,7 @@ struct usb_gadget_ops {
 	int	(*udc_start)(struct usb_gadget *,
 			struct usb_gadget_driver *);
 	int	(*udc_stop)(struct usb_gadget *);
+	void	(*udc_set_speed)(struct usb_gadget *, enum usb_device_speed);
 	struct usb_ep *(*match_ep)(struct usb_gadget *,
 			struct usb_endpoint_descriptor *,
 			struct usb_ss_ep_comp_descriptor *);