From 4f56f3fdca43c9a18339b6e0c3b1aa2f57f6d0b0 Mon Sep 17 00:00:00 2001 From: James Hogan Date: Fri, 25 Sep 2015 15:36:10 -0400 Subject: serial: 8250: Tolerate clock variance for max baud rate When the UART clock is set slightly under 1.8432MHz, the 8250 driver core doesn't permit the 115200 baud rate since it calculates the maximum frequency to pass to uart_get_baud_rate by simply dividing the uart clock by 16 which yields a value slightly under 115200, even though the frequency is close enough for the UART to operate reliably. Therefore add some tolerance in the calculation of the maximum baud rate. 1% tolerance allows for marginally slower uart clk than nominal without introducing transmission errors. Signed-off-by: James Hogan [pjh: Forward-port & refactor original patch; change tolerance to 1%] Signed-off-by: Peter Hurley Signed-off-by: Greg Kroah-Hartman --- drivers/tty/serial/8250/8250_port.c | 28 +++++++++++++++++++--------- 1 file changed, 19 insertions(+), 9 deletions(-) (limited to 'drivers/tty') diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c index 554a63e4c4a0..e54de0037df4 100644 --- a/drivers/tty/serial/8250/8250_port.c +++ b/drivers/tty/serial/8250/8250_port.c @@ -2225,6 +2225,23 @@ static void serial8250_set_divisor(struct uart_port *port, unsigned int baud, serial_port_out(port, 0x2, quot_frac); } +static unsigned int +serial8250_get_baud_rate(struct uart_port *port, struct ktermios *termios, + struct ktermios *old) +{ + unsigned int tolerance = port->uartclk / 100; + + /* + * Ask the core to calculate the divisor for us. + * Allow 1% tolerance at the upper limit so uart clks marginally + * slower than nominal still match standard baud rates without + * causing transmission errors. + */ + return uart_get_baud_rate(port, termios, old, + port->uartclk / 16 / 0xffff, + (port->uartclk + tolerance) / 16); +} + void serial8250_do_set_termios(struct uart_port *port, struct ktermios *termios, struct ktermios *old) @@ -2236,12 +2253,7 @@ serial8250_do_set_termios(struct uart_port *port, struct ktermios *termios, cval = serial8250_compute_lcr(up, termios->c_cflag); - /* - * Ask the core to calculate the divisor for us. - */ - baud = uart_get_baud_rate(port, termios, old, - port->uartclk / 16 / 0xffff, - port->uartclk / 16); + baud = serial8250_get_baud_rate(port, termios, old); quot = serial8250_get_divisor(up, baud, &frac); /* @@ -2834,9 +2846,7 @@ void serial8250_console_write(struct uart_8250_port *up, const char *s, if (port->state->port.tty && termios.c_cflag == 0) termios.c_cflag = port->state->port.tty->termios.c_cflag; - baud = uart_get_baud_rate(port, &termios, NULL, - port->uartclk / 16 / 0xffff, - port->uartclk / 16); + baud = serial8250_get_baud_rate(port, &termios, NULL); quot = serial8250_get_divisor(up, baud, &frac); serial8250_set_divisor(port, baud, quot, frac); -- cgit v1.2.3