From: James Hogan <james.hogan@xxxxxxxxxx>
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 <james.hogan@xxxxxxxxxx>
[pjh: Forward-port & refactor original patch; change tolerance to 1%]
Signed-off-by: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>
---
drivers/tty/serial/8250/8250_port.c | 28 +++++++++++++++++++---------
1 file changed, 19 insertions(+), 9 deletions(-)
diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index e34a37f..50e2633 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -2222,6 +2222,23 @@ 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)
@@ -2233,12 +2250,7 @@ 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);
/*
@@ -2840,9 +2852,7 @@ 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);
--
2.5.3
--
To unsubscribe from this list: send the line "unsubscribe linux-serial" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
