linux-kernel - [SERIAL] Don't optimise away baud rate changes when BOTHER is used

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Date:	Wed, 06 Jun 2007 13:30:10 +0100
From:	David Woodhouse <dwmw2@...radead.org>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	akpm@...l.org, linux-kernel@...r.kernel.org, paulus@...ba.org
Subject: [SERIAL] Don't optimise away baud rate changes when BOTHER is used

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().

Signed-off-by: David Woodhouse <dwmw2@...radead.org>

diff --git a/drivers/serial/serial_core.c b/drivers/serial/serial_core.c
index e8be3b5..a81fc08 100644
--- a/drivers/serial/serial_core.c
+++ b/drivers/serial/serial_core.c
@@ -1146,13 +1146,23 @@ 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)
+#else
+#define RELEVANT_CFLAG(cflag)	((cflag) & ~(CBAUD)
+#endif
 #define RELEVANT_IFLAG(iflag)	((iflag) & (IGNBRK|BRKINT|IGNPAR|PARMRK|INPCK))
-
 	if ((cflag ^ old_termios->c_cflag) == 0 &&
-	    RELEVANT_IFLAG(tty->termios->c_iflag ^ old_termios->c_iflag) == 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) {
+		printk("No relevant change\n");
 		return;
+	}
 
 	uart_change_speed(state, old_termios);
 

-- 
dwmw2

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