[PATCH] USB: serial: cp210x: Adding tx_empty() to avoid cp2108 failure

[Konstantin Shkolnyy <konstantin.shkolnyy@gmail.com>]

Occasionally, writing data and immediately closing the port makes cp2108
stop responding. The device had to be unplugged to clear the error.
The failure is induced by shutting down the device while its Tx queue still has
unsent data. Reporting the correct amount of those data avoids the problem.
Adding tx_empty() has no adverse effect on other cp210x devices.

Signed-off-by: Konstantin Shkolnyy <konstantin.shkolnyy@gmail.com>
---
 drivers/usb/serial/cp210x.c | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/drivers/usb/serial/cp210x.c b/drivers/usb/serial/cp210x.c
index eac7cca..0189e64 100644
--- a/drivers/usb/serial/cp210x.c
+++ b/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,
@@ -214,6 +215,7 @@ 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,
 	.attach			= cp210x_startup,
@@ -249,6 +251,16 @@ static struct usb_serial_driver * const serial_drivers[] = {
 #define CP210X_GET_CHARS	0x0E
 #define CP210X_GET_PROPS	0x0F
 #define CP210X_GET_COMM_STATUS	0x10
+/* Data returned by CP210X_GET_COMM_STATUS -- h/w doc says it's 0x13 bytes */
+struct cp210x_comm_status {
+	u32     errors;
+	u32     hold_reasons;
+	u32     amount_in_in_queue;
+	u32     amount_in_out_queue;
+	u8      eof_received;
+	u8      wait_for_immediate;
+	u8      reserved;
+};
 #define CP210X_RESET		0x11
 #define CP210X_PURGE		0x12
 #define CP210X_SET_FLOW		0x13
@@ -479,6 +491,24 @@ static void cp210x_close(struct usb_serial_port *port)
 	cp210x_set_config_single(port, CP210X_IFC_ENABLE, UART_DISABLE);
 }
 
+static bool cp210x_tx_empty(struct usb_serial_port *port)
+{
+	int err;
+
+	/* get_config needs "array of integers large enough", so pad to 0x14 bytes */
+	struct cp210x_comm_status_container {
+		struct cp210x_comm_status  sts; /* 0x13 bytes */
+		u8                         pad_to_0x14_bytes;
+	} comm_sts_cont;
+
+	err = cp210x_get_config(port, CP210X_GET_COMM_STATUS, (unsigned int *) &comm_sts_cont, 0x13);
+
+	if (!err)
+		if (comm_sts_cont.sts.amount_in_out_queue)
+			return false;
+	return true;
+}
+
 /*
  * cp210x_get_termios
  * Reads the baud rate, data bits, parity, stop bits and flow control mode
-- 
1.9.1