[PATCH V6 08/10] USB: f81232: fix read MSR strange value

[Peter Hung <hpeter@gmail.com>]

When we use RS232 loopback, assume doing RTS change will cause
CTS change, DTR change will cause DCD/DSR change too.

Sometimes we got 7~4 bits of MSR changed but the 3~0 bits of
MSR(delta) maybe not changed when set & get MCR fasterly.

So we add more check not only UART_MSR_ANY_DELTA but also with
comparing DCD/RI/DSR/CTS change with old value. Due to the state
bit is always correct, we direct save msr when read.

The following step to reproduce this problem with while loop step 1~4:
1. ioctl(fd, TIOCMSET, &data) to set RTS or DTR
2. ioctl(fd, TIOCMGET, &data) to read CTS or DCD/DSR state
3. ioctl(fd, TIOCMSET, &data) to unset RTS or DTR
4. ioctl(fd, TIOCMGET, &data) to read CTS or DCD/DSR state

Signed-off-by: Peter Hung <hpeter+linux_kernel@gmail.com>
---
 drivers/usb/serial/f81232.c | 23 +++++++++++++++++------
 1 file changed, 17 insertions(+), 6 deletions(-)

diff --git a/drivers/usb/serial/f81232.c b/drivers/usb/serial/f81232.c
index c87a3eb..94c05d7 100644
--- a/drivers/usb/serial/f81232.c
+++ b/drivers/usb/serial/f81232.c
@@ -110,7 +110,8 @@ static void f81232_read_msr(struct usb_serial_port *port)
 {
 	int status;
 	unsigned long flags;
-	u8 current_msr;
+	u8 current_msr, prev_msr;
+	u8 msr_mask = ~UART_MSR_ANY_DELTA;
 	struct tty_struct *tty;
 	struct f81232_private *priv = usb_get_serial_port_data(port);
 
@@ -123,7 +124,21 @@ static void f81232_read_msr(struct usb_serial_port *port)
 		return;
 	}
 
-	if (!(current_msr & UART_MSR_ANY_DELTA))
+	/*
+	*  The 7~4 bits of MSR will change but the 3~0 bits of MSR(delta)
+	*  maybe not change when set & get MCR fasterly.
+	*
+	*  So we add more check with comparing DCD/RI/DSR/CTS
+	*  change. and direct save msr when read.
+	*/
+
+	spin_lock_irqsave(&priv->lock, flags);
+	prev_msr = priv->modem_status;
+	priv->modem_status = current_msr;
+	spin_unlock_irqrestore(&priv->lock, flags);
+
+	if (!(current_msr & UART_MSR_ANY_DELTA) &&
+		!((prev_msr ^ current_msr) & msr_mask))
 		return;
 
 	tty = tty_port_tty_get(&port->port);
@@ -136,10 +151,6 @@ static void f81232_read_msr(struct usb_serial_port *port)
 		tty_kref_put(tty);
 	}
 
-	spin_lock_irqsave(&priv->lock, flags);
-	priv->modem_status = current_msr;
-	spin_unlock_irqrestore(&priv->lock, flags);
-
 	wake_up_interruptible(&port->port.delta_msr_wait);
 }
 
-- 
1.9.1