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Subject[SERIAL] Don't optimise away baud rate changes when BOTHER is used (v2)
The uart_set_termios() function will bail out early without bothering to
touch the hardware, if it decides that nothing "relevant" has changed.
Unfortunately, its idea of "relevant" doesn't include c_[io]speed. So if
the baud rate bits are BOTHER and you just change the speed, the change
gets optimised away.

This patch makes it ignore the old Bfoo bits in c_cflag and just check
whether c_ispeed and c_ospeed have changed. Those integers are always
set appropriately for us by set_termios().

This version of the patch lacks the debugging printk which I
accidentally left in the previous one.

Signed-off-by: David Woodhouse <>

diff --git a/drivers/serial/serial_core.c b/drivers/serial/serial_core.c
index e8be3b5..4480990 100644
--- a/drivers/serial/serial_core.c
+++ b/drivers/serial/serial_core.c
@@ -1146,11 +1146,19 @@ static void uart_set_termios(struct tty_struct *tty, struct ktermios *old_termio

* These are the bits that are used to setup various
- * flags in the low level driver.
+ * flags in the low level driver. We can ignore the Bfoo
+ * bits in c_cflag; c_[io]speed will always be set
+ * appropriately by set_termios() in tty_ioctl.c
+#ifdef CIBAUD
+#define RELEVANT_CFLAG(cflag) ((cflag) & ~(CIBAUD|CBAUD)
+#define RELEVANT_CFLAG(cflag) ((cflag) & ~(CIBAUD|CBAUD)
if ((cflag ^ old_termios->c_cflag) == 0 &&
+ tty->termios->c_ospeed == old_termios->c_ospeed &&
+ tty->termios->c_ispeed == old_termios->c_ispeed &&
RELEVANT_IFLAG(tty->termios->c_iflag ^ old_termios->c_iflag) == 0)


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