USB: pl2303: fix data corruption on termios updates

Some PL2303 devices are known to lose bytes if you change serial
settings even to the same values as before. Avoid this by comparing the
encoded settings with the previsouly used ones before configuring the
device.

The common case was fixed by commit bf5e5834bffc6 ("pl2303: Fix mode
switching regression"), but this problem was still possible to trigger,
for instance, by using the TCSETS2-interface to repeatedly request
115201 baud, which gets mapped to 115200 and thus always triggers a
settings update.

Cc: Frank Schäfer <fschaefer.oss@googlemail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/serial/pl2303.c

@@ -142,6 +142,8 @@ struct pl2303_private {
 	spinlock_t lock;
 	u8 line_control;
 	u8 line_status;
+
+	u8 line_settings[7];
 };
 
 static int pl2303_vendor_read(__u16 value, __u16 index,
@@ -339,11 +341,6 @@ static void pl2303_set_termios(struct tty_struct *tty,
 	int i;
 	u8 control;
 
-	/*
-	 * The PL2303 is reported to lose bytes if you change serial settings
-	 * even to the same values as before. Thus we actually need to filter
-	 * in this specific case.
-	 */
 	if (old_termios && !tty_termios_hw_change(&tty->termios, old_termios))
 		return;
 
@@ -428,10 +425,29 @@ static void pl2303_set_termios(struct tty_struct *tty,
 		dev_dbg(&port->dev, "parity = none\n");
 	}
 
-	i = usb_control_msg(serial->dev, usb_sndctrlpipe(serial->dev, 0),
-			    SET_LINE_REQUEST, SET_LINE_REQUEST_TYPE,
-			    0, 0, buf, 7, 100);
-	dev_dbg(&port->dev, "0x21:0x20:0:0  %d\n", i);
+	/*
+	 * Some PL2303 are known to lose bytes if you change serial settings
+	 * even to the same values as before. Thus we actually need to filter
+	 * in this specific case.
+	 *
+	 * Note that the tty_termios_hw_change check above is not sufficient
+	 * as a previously requested baud rate may differ from the one
+	 * actually used (and stored in old_termios).
+	 *
+	 * NOTE: No additional locking needed for line_settings as it is
+	 *       only used in set_termios, which is serialised against itself.
+	 */
+	if (!old_termios || memcmp(buf, priv->line_settings, 7)) {
+		i = usb_control_msg(serial->dev,
+				    usb_sndctrlpipe(serial->dev, 0),
+				    SET_LINE_REQUEST, SET_LINE_REQUEST_TYPE,
+				    0, 0, buf, 7, 100);
+
+		dev_dbg(&port->dev, "0x21:0x20:0:0  %d\n", i);
+
+		if (i == 7)
+			memcpy(priv->line_settings, buf, 7);
+	}
 
 	/* change control lines if we are switching to or from B0 */
 	spin_lock_irqsave(&priv->lock, flags);