Implement USB CLEAR_FEATURE request for ENDPOINT_HALT.

From USB 2.0, section 9.4.5:

"For endpoints using data toggle, regardless of whether an endpoint has
the Halt feature set, a ClearFeature(ENDPOINT_HALT) request always
results in the data toggle being reinitialized to DATA0."

firmware/common/usb.c

@@ -302,6 +302,17 @@ void usb_endpoint_stall(
 	// TODO: Also need to reset data toggle in both directions?
 }
 
+void usb_endpoint_reset_data_toggle(
+	const usb_endpoint_t* const endpoint
+) {
+	const uint_fast8_t endpoint_number = usb_endpoint_number(endpoint->address);
+	if( usb_endpoint_is_in(endpoint->address) ) {
+		USB0_ENDPTCTRL(endpoint_number) |= USB0_ENDPTCTRL_TXR;
+	} else {
+		USB0_ENDPTCTRL(endpoint_number) |= USB0_ENDPTCTRL_RXR;
+	}
+}
+
 static void usb_controller_run() {
 	USB0_USBCMD_D |= USB0_USBCMD_D_RS;
 }

firmware/common/usb.h

@@ -70,6 +70,10 @@ void usb_endpoint_disable(
 	const usb_endpoint_t* const endpoint
 );
 
+void usb_endpoint_reset_data_toggle(
+	const usb_endpoint_t* const endpoint
+);
+
 void usb_endpoint_flush(
 	const usb_endpoint_t* const endpoint
 );

firmware/common/usb_standard_request.c

@@ -333,7 +333,6 @@ static usb_request_status_t usb_standard_request_get_configuration(
 	}
 }
 
-
 static usb_request_status_t usb_standard_request_get_status_setup(
 	usb_endpoint_t* const endpoint
 ) {
@@ -367,6 +366,35 @@ static usb_request_status_t usb_standard_request_get_status(
 	}
 }
 
+static usb_request_status_t usb_standard_request_clear_feature_setup(
+	usb_endpoint_t* const endpoint)
+{
+	switch (endpoint->setup.value) {
+		case USB_FEATURE_SELECTOR_ENDPOINT_HALT:
+			usb_endpoint_reset_data_toggle(endpoint);
+			return USB_REQUEST_STATUS_OK;
+		default:
+			return USB_REQUEST_STATUS_STALL;
+	}
+}
+
+static usb_request_status_t usb_standard_request_clear_feature(
+	usb_endpoint_t* const endpoint,
+	const usb_transfer_stage_t stage)
+{
+	switch (stage) {
+	case USB_TRANSFER_STAGE_SETUP:
+		return usb_standard_request_clear_feature_setup(endpoint);
+
+	case USB_TRANSFER_STAGE_DATA:
+	case USB_TRANSFER_STAGE_STATUS:
+		return USB_REQUEST_STATUS_OK;
+
+	default:
+		return USB_REQUEST_STATUS_STALL;
+	}
+}
+
 /*********************************************************************/
 
 usb_request_status_t usb_standard_request(
@@ -389,6 +417,9 @@ usb_request_status_t usb_standard_request(
 	case USB_STANDARD_REQUEST_GET_CONFIGURATION:
 		return usb_standard_request_get_configuration(endpoint, stage);
 
+	case USB_STANDARD_REQUEST_CLEAR_FEATURE:
+		return usb_standard_request_clear_feature(endpoint, stage);
+
 	default:
 		return USB_REQUEST_STATUS_STALL;
 	}

firmware/common/usb_type.h

@@ -70,6 +70,10 @@ typedef enum {
 	USB_STANDARD_REQUEST_SYNCH_FRAME = 12,
 } usb_standard_request_t;
 
+typedef enum {
+	USB_FEATURE_SELECTOR_ENDPOINT_HALT = 0,
+} usb_feature_selector_t;
+
 typedef enum {
 	USB_SETUP_REQUEST_TYPE_shift = 5,
 	USB_SETUP_REQUEST_TYPE_mask = 3 << USB_SETUP_REQUEST_TYPE_shift,