pl2303: Fix mode switching regression

Cleaning out all the incorrect 'no change made' checks for termios
settings showed up a problem with the PL2303. The hardware here seems to
lose sync and bits if you tell it to make no changes. This shows up with
a real world application.

To fix this the driver check for meaningful hardware changes is restored
but doing the tests correctly and as a tty layer function so it doesn't
get duplicated wrongly everywhere if other drivers turn out to need it.

Signed-off-by: Alan Cox <alan@redhat.com>
Tested-by: Mirko Parthey <mirko.parthey@informatik.tu-chemnitz.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 7 insertions, 0 deletions

diff --git a/drivers/usb/serial/pl2303.c b/drivers/usb/serial/pl2303.c
index cf8add91de05..0da1df9c79bf 100644
--- a/drivers/usb/serial/pl2303.c
+++ b/drivers/usb/serial/pl2303.c
@@ -483,6 +483,13 @@ static void pl2303_set_termios(struct usb_serial_port *port,
         }
         spin_unlock_irqrestore(&priv->lock, flags);
 
+        /* 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 (!tty_termios_hw_change(port->tty->termios, old_termios))
+                return;
+
         cflag = port->tty->termios->c_cflag;
 
         buf = kzalloc(7, GFP_KERNEL);