On Wed, 2021-05-05 at 11:19 +0200, Jiri Slaby wrote:

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.

[]

[]

+/**
+ *	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 = account_flags ? 2 : 0;
+
+	/* byte size and parity */
+	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 && (cflag & CSTOPB))
+		bits++;
+
+	if (account_flags && (cflag & PARENB))
+		bits++;
+
+	return bits;
+}
+EXPORT_SYMBOL_GPL(tty_get_byte_size);

Perhaps clearer not testing account_flags multiple times.

unsigned char tty_get_byte_size(unsigned int cflag, bool account_flags)
{
	unsigned char bits;

	/* byte size and parity */
	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) {
		bits += 2;	/* start/stop bits */

		if (cflag & CSTOPB)
			bits++;

		if (cflag & PARENB)
			bits++;
	}

	return bits;
}