[PATCH v2] staging: greybus: uart: clear unsupported c_cflag bits in set_termios

[Debjeet Banerjee <debjeetbanerjee48@gmail.com>]

gb_tty_set_termios() derives UART line configuration from a subset of
termios->c_cflag bits (CSIZE, CSTOPB, PARENB, PARODD, CMSPAR, CRTSCTS,
CLOCAL and CBAUD). Other bits are not interpreted by the driver and are
not represented in the Greybus UART protocol.

Mask unsupported c_cflag bits so that userspace-visible termios
reflects the supported bits implemented by the driver.

This addresses the existing FIXME.

Signed-off-by: Debjeet Banerjee <debjeetbanerjee48@gmail.com>
---
v2:
  - Clear unsupported c_cflag bits as suggested
  - Update comment to mention the change
---
 drivers/staging/greybus/uart.c | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/drivers/staging/greybus/uart.c b/drivers/staging/greybus/uart.c
index 7d060b4cd33d..9c10e6baa7e0 100644
--- a/drivers/staging/greybus/uart.c
+++ b/drivers/staging/greybus/uart.c
@@ -495,7 +495,24 @@ static void gb_tty_set_termios(struct tty_struct *tty,
 
 	newline.data_bits = tty_get_char_size(termios->c_cflag);
 
-	/* FIXME: needs to clear unsupported bits in the termios */
+	/*
+	 * The Greybus UART driver only interprets a subset of termios
+	 * c_cflag bits when configuring line settings:
+	 *
+	 *   - CSIZE via tty_get_char_size() for data bits
+	 *   - CSTOPB for stop-bit format
+	 *   - PARENB, PARODD, CMSPAR for parity encoding
+	 *   - CRTSCTS for hardware flow control
+	 *   - CLOCAL for modem control handling
+	 *   - CBAUD via C_BAUD() for baud rate and B0 semantics
+	 *
+	 * Mask unsupported c_cflag bits.
+	 */
+	termios->c_cflag &= (CSIZE | CSTOPB |
+			     PARENB | PARODD | CMSPAR |
+			     CLOCAL | CRTSCTS |
+			     CBAUD);
+
 	gb_tty->clocal = ((termios->c_cflag & CLOCAL) != 0);
 
 	if (C_BAUD(tty) == B0) {
-- 
2.53.0