[PATCH v3] USB: serial: cp210x: Fix error 32 when hardware flow control is enabled.

From: Pho Tran
Date: Thu Jan 21 2021 - 02:13:51 EST

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>
01/19/2021: 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..5b6bbda2b424 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);
+ if (C_CRTSCTS(tty))
+ /* Don't send SET_MHS command if device in hardware flow control mode. */
+ return 0;
+ else
+ return cp210x_tiocmset_port(port, set, clear);

static int cp210x_tiocmset_port(struct usb_serial_port *port,