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>

2 files changed, 26 insertions, 0 deletions

diff --git a/drivers/char/tty_ioctl.c b/drivers/char/tty_ioctl.c
index e02d59245a17..d4b6d64e858b 100644
--- a/drivers/char/tty_ioctl.c
+++ b/drivers/char/tty_ioctl.c
@@ -365,6 +365,25 @@ void tty_termios_copy_hw(struct ktermios *new, struct ktermios *old)
 EXPORT_SYMBOL(tty_termios_copy_hw);
 
 /**
+ *      tty_termios_hw_change   -       check for setting change
+ *      @a: termios
+ *      @b: termios to compare
+ *
+ *      Check if any of the bits that affect a dumb device have changed
+ *      between the two termios structures, or a speed change is needed.
+ */
+
+int tty_termios_hw_change(struct ktermios *a, struct ktermios *b)
+{
+        if (a->c_ispeed != b->c_ispeed || a->c_ospeed != b->c_ospeed)
+                return 1;
+        if ((a->c_cflag ^ b->c_cflag) & ~(HUPCL | CREAD | CLOCAL))
+                return 1;
+        return 0;
+}
+EXPORT_SYMBOL(tty_termios_hw_change);
+
+/**
  *      change_termios          -       update termios values
  *      @tty: tty to update
  *      @new_termios: desired new value
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);