[PATCH v4] USB: serial: cp210x: Fix error 32 when hardware flow control is enabled.

[Pho Tran]

Fix error 32 returned by CP210X_SET_MHS when hardware flow control is enabled.

The root cause of error 32 is that user application (CoolTerm, linux-serial-test)
opened cp210x device with hardware flow control then attempt to control RTS/DTR pins.
In hardware flow control, RTS/DTR pins will be controlled by hardware only,
any attempt to control those pins will cause error 32 from the device.
This fix will block MHS command(command to control RTS/DTR pins) to the device
if hardware flow control is being used.

Signed-off-by: Pho Tran <pho.tran@xxxxxxxxxx>
---
01/21/2021: Patch v3 modified based on comment from Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
01/19/2021: Patch v2  Modified based on comment from Johan Hovold <johan@xxxxxxxxxx>
and Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 drivers/usb/serial/cp210x.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/usb/serial/cp210x.c b/drivers/usb/serial/cp210x.c
index fbb10dfc56e3..814dff5fee98 100644
--- a/drivers/usb/serial/cp210x.c
+++ b/drivers/usb/serial/cp210x.c
@@ -1204,7 +1204,12 @@ static int cp210x_tiocmset(struct tty_struct *tty,
 		unsigned int set, unsigned int clear)
 {
 	struct usb_serial_port *port = tty->driver_data;
-	return cp210x_tiocmset_port(port, set, clear);
+
+	/* Don't send SET_MHS command if device in hardware flow control mode. */
+	if (C_CRTSCTS(tty))
+		return 0;
+	else
+		return cp210x_tiocmset_port(port, set, clear);
 }
 
 static int cp210x_tiocmset_port(struct usb_serial_port *port,
-- 
2.17.1