greybus: uart: Implement flush_buffer

Data may be held pening in the hardware because of flow
control mechanisms. When the port is closed, we need to flush
all data that was not sent.

For this, use the greybus message GB_UART_TYPE_FLUSH_FIFOS
which will flush all data queued on the module but not
yet sent on the data line.
Suggested-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Axel Haslam <ahaslam@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>

drivers/staging/greybus/greybus_protocols.h

@@ -1270,6 +1270,7 @@ struct gb_raw_send_request {
 #define GB_UART_TYPE_SEND_BREAK			0x06
 #define GB_UART_TYPE_SERIAL_STATE		0x07	/* Unsolicited data */
 #define GB_UART_TYPE_RECEIVE_CREDITS		0x08
+#define GB_UART_TYPE_FLUSH_FIFOS		0x09
 
 /* Represents data from AP -> Module */
 struct gb_uart_send_data_request {
@@ -1335,6 +1336,12 @@ struct gb_uart_serial_state_request {
 	__u8	control;
 } __packed;
 
+struct gb_uart_serial_flush_request {
+	__u8    flags;
+#define GB_SERIAL_FLAG_FLUSH_TRANSMITTER	0x01
+#define GB_SERIAL_FLAG_FLUSH_RECEIVER		0x02
+} __packed;
+
 /* Loopback */
 
 /* Version of the Greybus loopback protocol we support */

drivers/staging/greybus/uart.c

@@ -317,6 +317,14 @@ static int send_break(struct gb_tty *gb_tty, u8 state)
 				 &request, sizeof(request), NULL, 0);
 }
 
+static int gb_uart_flush(struct gb_tty *gb_tty, u8 flags)
+{
+	struct gb_uart_serial_flush_request request;
+
+	request.flags = flags;
+	return gb_operation_sync(gb_tty->connection, GB_UART_TYPE_FLUSH_FIFOS,
+				 &request, sizeof(request), NULL, 0);
+}
 
 static struct gb_tty *get_gb_by_minor(unsigned minor)
 {
@@ -745,6 +753,7 @@ static void gb_tty_port_shutdown(struct tty_port *port)
 {
 	struct gb_tty *gb_tty;
 	unsigned long flags;
+	int ret;
 
 	gb_tty = container_of(port, struct gb_tty, port);
 
@@ -756,6 +765,12 @@ static void gb_tty_port_shutdown(struct tty_port *port)
 	kfifo_reset_out(&gb_tty->write_fifo);
 	spin_unlock_irqrestore(&gb_tty->write_lock, flags);
 
+	ret = gb_uart_flush(gb_tty, GB_SERIAL_FLAG_FLUSH_TRANSMITTER);
+	if (ret) {
+		dev_err(&gb_tty->gbphy_dev->dev,
+			"error flushing transmitter: %d\n", ret);
+	}
+
 	gb_tty->close_pending = false;
 }