Many tty drivers contain code to compute bits count depending on termios
cflags. So extract this code from serial core to a separate tty helper
function called tty_get_byte_size.

In the next patch, call to this new function will replace many copies of
this code.

[v2] simplified the code flow as suggested by Joe and Andy

Signed-off-by: Jiri Slaby <redacted>
Reviewed-by: Andy Shevchenko <redacted>
Cc: Joe Perches <joe@perches.com>
---
 drivers/tty/serial/serial_core.c | 30 +++--------------------
 drivers/tty/tty_ioctl.c          | 42 ++++++++++++++++++++++++++++++++
 include/linux/tty.h              |  2 ++
 3 files changed, 47 insertions(+), 27 deletions(-)

diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index d29329eb52f4..b3fc2b02a705 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c

@@ -334,39 +334,15 @@ void
 uart_update_timeout(struct uart_port *port, unsigned int cflag,
 		    unsigned int baud)
 {
-	unsigned int bits;
+	unsigned int size;
 
-	/* byte size and parity */
-	switch (cflag & CSIZE) {
-	case CS5:
-		bits = 7;
-		break;
-	case CS6:
-		bits = 8;
-		break;
-	case CS7:
-		bits = 9;
-		break;
-	default:
-		bits = 10;
-		break; /* CS8 */
-	}
-
-	if (cflag & CSTOPB)
-		bits++;
-	if (cflag & PARENB)
-		bits++;
-
-	/*
-	 * The total number of bits to be transmitted in the fifo.
-	 */
-	bits = bits * port->fifosize;
+	size = tty_get_byte_size(cflag, true) * port->fifosize;

 	/*
 	 * Figure the timeout to send the above number of bits.
 	 * Add .02 seconds of slop
 	 */
-	port->timeout = (HZ * bits) / baud + HZ/50;
+	port->timeout = (HZ * size) / baud + HZ/50;
 }
 
 EXPORT_SYMBOL(uart_update_timeout);

diff --git a/drivers/tty/tty_ioctl.c b/drivers/tty/tty_ioctl.c
index aa9ecc8be990..13acc3decd87 100644
--- a/drivers/tty/tty_ioctl.c
+++ b/drivers/tty/tty_ioctl.c

@@ -300,6 +300,48 @@ int tty_termios_hw_change(const struct ktermios *a, const struct ktermios *b)
 }
 EXPORT_SYMBOL(tty_termios_hw_change);
 
+/**
+ *	tty_get_byte_size	-	get size of a byte
+ *	@cflag: termios cflag value
+ *	@account_flags: account for start and stop bits, second stop bit (if
+ *			set), and parity (if set)
+ *
+ *	Get the size of a byte in bits depending on @cflag. Depending on
+ *	@account_flags parameter, the result also accounts start and stop bits,
+ *	the second stop bit, and parity bit.
+ */
+unsigned char tty_get_byte_size(unsigned int cflag, bool account_flags)
+{
+	unsigned char bits;
+
+	switch (cflag & CSIZE) {
+	case CS5:
+		bits = 5;
+		break;
+	case CS6:
+		bits = 6;
+		break;
+	case CS7:
+		bits = 7;
+		break;
+	case CS8:
+	default:
+		bits = 8;
+		break;
+	}
+
+	if (!account_flags)
+		return bits;
+
+	if (cflag & CSTOPB)
+		bits++;
+	if (cflag & PARENB)
+		bits++;
+
+	return bits + 2;
+}
+EXPORT_SYMBOL_GPL(tty_get_byte_size);