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, specifically +1/128th (~0.8%), to allow the 115200 baud rate to be closen for a UART clock as low as 1.8149MHz. For an external divider set as close as possible to 1.8432MHz, this should cover every possible input frequency above 118.9MHz. Signed-off-by: James Hogan <james.hogan@xxxxxxxxxx> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> Cc: Jiri Slaby <jslaby@xxxxxxx> Cc: Anton Vorontsov <avorontsov@xxxxxxxxxxxxx> Cc: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx> Cc: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> Cc: linux-serial@xxxxxxxxxxxxxxx --- As far as I can tell from reading the link below, this tolerance should be okay, and it definitely covers the range of input frequencies I expect for my particular hardware. I'm open to better ways of handling it though. https://electronics.stackexchange.com/questions/5850/how-critical-are-uart-frequencies --- drivers/tty/serial/8250/8250_core.c | 13 ++++++++----- 1 file changed, 8 insertions(+), 5 deletions(-) diff --git a/drivers/tty/serial/8250/8250_core.c b/drivers/tty/serial/8250/8250_core.c index e3b9570a..5d3d171 100644 --- a/drivers/tty/serial/8250/8250_core.c +++ b/drivers/tty/serial/8250/8250_core.c @@ -2537,16 +2537,18 @@ serial8250_do_set_termios(struct uart_port *port, struct ktermios *termios, struct uart_8250_port *up = up_to_u8250p(port); unsigned char cval; unsigned long flags; - unsigned int baud, quot, frac = 0; + unsigned int baud, quot, frac = 0, uartclk_hi; cval = serial8250_compute_lcr(up, termios->c_cflag); /* - * Ask the core to calculate the divisor for us. + * Ask the core to calculate the divisor for us, but tolerate some + * variance (e.g. if the uartclk is only slightly under 1.8432MHz). */ + uartclk_hi = port->uartclk + port->uartclk / 128; baud = uart_get_baud_rate(port, termios, old, port->uartclk / 16 / 0xffff, - port->uartclk / 16); + uartclk_hi / 16); quot = serial8250_get_divisor(up, baud, &frac); /* @@ -3270,15 +3272,16 @@ serial8250_console_write(struct console *co, const char *s, unsigned int count) /* check scratch reg to see if port powered off during system sleep */ if (up->canary && (up->canary != serial_port_in(port, UART_SCR))) { struct ktermios termios; - unsigned int baud, quot, frac = 0; + unsigned int baud, quot, frac = 0, uartclk_hi; termios.c_cflag = port->cons->cflag; if (port->state->port.tty && termios.c_cflag == 0) termios.c_cflag = port->state->port.tty->termios.c_cflag; + uartclk_hi = port->uartclk + port->uartclk / 128; baud = uart_get_baud_rate(port, &termios, NULL, port->uartclk / 16 / 0xffff, - port->uartclk / 16); + uartclk_hi / 16); quot = serial8250_get_divisor(up, baud, &frac); serial8250_set_divisor(port, baud, quot, frac); -- 2.0.5 -- 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
