USB: cp210x: add tx_empty()

Added tx_empty callback needed for generic wait-until-sent support.
Without this function, when the port is closed usbserial can't know that
there are still data in the chip's transmit FIFO. The chip gets disabled
and untransmitted data lost. When the actual byte count is reported by
tx-empty the close can be delayed until all data are sent.

Signed-off-by: Konstantin Shkolnyy <konstantin.shkolnyy@gmail.com>
[johan: modify tx_empty error handling ]
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/cp210x.c

@@ -38,6 +38,7 @@ static void cp210x_change_speed(struct tty_struct *, struct usb_serial_port *,
 							struct ktermios *);
 static void cp210x_set_termios(struct tty_struct *, struct usb_serial_port *,
 							struct ktermios*);
+static bool cp210x_tx_empty(struct usb_serial_port *port);
 static int cp210x_tiocmget(struct tty_struct *);
 static int cp210x_tiocmset(struct tty_struct *, unsigned int, unsigned int);
 static int cp210x_tiocmset_port(struct usb_serial_port *port,
@@ -215,6 +216,7 @@ static struct usb_serial_driver cp210x_device = {
 	.close			= cp210x_close,
 	.break_ctl		= cp210x_break_ctl,
 	.set_termios		= cp210x_set_termios,
+	.tx_empty		= cp210x_tx_empty,
 	.tiocmget		= cp210x_tiocmget,
 	.tiocmset		= cp210x_tiocmset,
 	.port_probe		= cp210x_port_probe,
@@ -301,6 +303,17 @@ static struct usb_serial_driver * const serial_drivers[] = {
 #define CONTROL_WRITE_DTR	0x0100
 #define CONTROL_WRITE_RTS	0x0200
 
+/* CP210X_GET_COMM_STATUS returns these 0x13 bytes */
+struct cp210x_comm_status {
+	__le32   ulErrors;
+	__le32   ulHoldReasons;
+	__le32   ulAmountInInQueue;
+	__le32   ulAmountInOutQueue;
+	u8       bEofReceived;
+	u8       bWaitForImmediate;
+	u8       bReserved;
+} __packed;
+
 /*
  * CP210X_PURGE - 16 bits passed in wValue of USB request.
  * SiLabs app note AN571 gives a strange description of the 4 bits:
@@ -549,6 +562,51 @@ static void cp210x_close(struct usb_serial_port *port)
 	cp210x_set_config_single(port, CP210X_IFC_ENABLE, UART_DISABLE);
 }
 
+/*
+ * Read how many bytes are waiting in the TX queue.
+ */
+static int cp210x_get_tx_queue_byte_count(struct usb_serial_port *port,
+		u32 *count)
+{
+	struct usb_serial *serial = port->serial;
+	struct cp210x_port_private *port_priv = usb_get_serial_port_data(port);
+	struct cp210x_comm_status *sts;
+	int result;
+
+	sts = kmalloc(sizeof(*sts), GFP_KERNEL);
+	if (!sts)
+		return -ENOMEM;
+
+	result = usb_control_msg(serial->dev, usb_rcvctrlpipe(serial->dev, 0),
+			CP210X_GET_COMM_STATUS, REQTYPE_INTERFACE_TO_HOST,
+			0, port_priv->bInterfaceNumber, sts, sizeof(*sts),
+			USB_CTRL_GET_TIMEOUT);
+	if (result == sizeof(*sts)) {
+		*count = le32_to_cpu(sts->ulAmountInOutQueue);
+		result = 0;
+	} else {
+		dev_err(&port->dev, "failed to get comm status: %d\n", result);
+		if (result >= 0)
+			result = -EPROTO;
+	}
+
+	kfree(sts);
+
+	return result;
+}
+
+static bool cp210x_tx_empty(struct usb_serial_port *port)
+{
+	int err;
+	u32 count;
+
+	err = cp210x_get_tx_queue_byte_count(port, &count);
+	if (err)
+		return true;
+
+	return !count;
+}
+
 /*
  * cp210x_get_termios
  * Reads the baud rate, data bits, parity, stop bits and flow control mode