[PATCH 2/6] USB: serial: mxuport: initialize modem status on open

[Crescent Hsieh]

The cached modem status is currently cleared whenever a port is opened,
regardless of the current modem-status inputs. Consequently, TIOCMGET
may report asserted input signals as inactive immediately after open.

Read the one-byte MSR with RQ_VENDOR_GET_MSR after setting up the port.
Ignore the transient delta bits and cache only the current modem-line
state.

If the request fails, keep the cached state clear and allow the port to
open, preserving the existing fallback behaviour.

Signed-off-by: Crescent Hsieh <crescentcy.hsieh@xxxxxxxx>
---
 drivers/usb/serial/mxuport.c | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/drivers/usb/serial/mxuport.c b/drivers/usb/serial/mxuport.c
index fa75f844050d..3f1989afde30 100644
--- a/drivers/usb/serial/mxuport.c
+++ b/drivers/usb/serial/mxuport.c
@@ -1452,6 +1452,7 @@ static int mxuport_open(struct tty_struct *tty, struct usb_serial_port *port)
 	struct mxuport_port *mxport = usb_get_serial_port_data(port);
 	struct usb_serial *serial = port->serial;
 	unsigned long flags;
+	u8 msr_state;
 	int err;
 
 	/* Set receive host (enable) */
@@ -1477,13 +1478,19 @@ static int mxuport_open(struct tty_struct *tty, struct usb_serial_port *port)
 	mxport->hold_reason &= ~MX_WAIT_FOR_SEND_NEXT;
 	spin_unlock_irqrestore(&port->lock, flags);
 
-	/*
-	 * TODO: use RQ_VENDOR_GET_MSR, once we know what it
-	 * returns.
-	 */
-	mxport->msr_state = 0;
+	err = mxuport_recv_ctrl(serial, RQ_VENDOR_GET_MSR, 0,
+				port->port_number, &msr_state,
+				sizeof(msr_state));
+	if (err)
+		msr_state = 0;
+	else
+		msr_state &= ~UART_MSR_ANY_DELTA;
 
-	return err;
+	spin_lock_irqsave(&mxport->spinlock, flags);
+	mxport->msr_state = msr_state;
+	spin_unlock_irqrestore(&mxport->spinlock, flags);
+
+	return 0;
 }
 
 static void mxuport_close(struct usb_serial_port *port)
-- 
2.53.0