serial: 8250: 8250_omap: Fix calculation of RS485 delays

Commit 801954d1210a ("serial: 8250: 8250_omap: Support native RS485")
calculates RS485 delays from the baudrate.  The baudrate is generated
with either a 16x or 13x divisor.  The divisor is set in the Mode
Definition Register 1 (MDR1).

The commit erroneously assumes that the register stores the divisor as
a bitmask and uses a logical AND to differentiate between 16x and 13x
divisors.  However the divisor is really stored as a 3-bit mode
(see lines 363ff in include/uapi/linux/serial_reg.h).

The logical AND operation is performed with UART_OMAP_MDR1_16X_MODE,
which is defined as 0x0 and hence yields false.  So the commit always
assumes a 13x divisor.  Fix by using an equal comparison.  This works
because we never set any of the other 5 bits in the register.  (They
pertain to IrDA mode, which is not supported by the driver).

Fixes: 801954d1210a ("serial: 8250: 8250_omap: Support native RS485")
Link: https://lore.kernel.org/linux-serial/202211070440.8hWunFUN-lkp@intel.com/
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/7d5b04da13d89b8708b9543a0b125f2b6062a77b.1667977259.git.lukas@wunner.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/serial/8250/8250_omap.c b/drivers/tty/serial/8250/8250_omap.c
index 1c8a48fdc8f24479ed864987613d4a2b72320b5a..7bb9da7558a1d1a11fc00242733966ce5ef2e6b1 100644 (file)
--- a/drivers/tty/serial/8250/8250_omap.c
+++ b/drivers/tty/serial/8250/8250_omap.c
@@ -822,7 +822,7 @@ static int omap8250_rs485_config(struct uart_port *port,
         * of the AM65 TRM:  https://www.ti.com/lit/ug/spruid7e/spruid7e.pdf
         */
        if (priv->quot) {
-               if (priv->mdr1 & UART_OMAP_MDR1_16X_MODE)
+               if (priv->mdr1 == UART_OMAP_MDR1_16X_MODE)
                        baud = port->uartclk / (16 * priv->quot);
                else
                        baud = port->uartclk / (13 * priv->quot);