serial: sc16is7xx: fix bug when first setting GPIO direction

When configuring a pin as an output pin with a value of logic 0, we
end up as having a value of logic 1 on the output pin. Setting a
logic 0 a second time (or more) after that will correctly output a
logic 0 on the output pin.

By default, all GPIO pins are configured as inputs. When we enter
sc16is7xx_gpio_direction_output() for the first time, we first set the
desired value in IOSTATE, and then we configure the pin as an output.
The datasheet states that writing to IOSTATE register will trigger a
transfer of the value to the I/O pin configured as output, so if the
pin is configured as an input, nothing will be transferred.

Therefore, set the direction first in IODIR, and then set the desired
value in IOSTATE.

This is what is done in NXP application note AN10587.

Fixes: dfeae619d781 ("serial: sc16is7xx")
Cc: stable@vger.kernel.org
Signed-off-by: Hugo Villeneuve <hvilleneuve@dimonoff.com>
Reviewed-by: Lech Perczak <lech.perczak@camlingroup.com>
Tested-by: Lech Perczak <lech.perczak@camlingroup.com>
Link: https://lore.kernel.org/r/20230807214556.540627-6-hugo@hugovil.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/serial/sc16is7xx.c b/drivers/tty/serial/sc16is7xx.c
index b45e34e3910dbfbaddf76a8feced008171a26ad3..cfc88959b2f78bf63c28bc4d67e53ae2d39def7a 100644 (file)
--- a/drivers/tty/serial/sc16is7xx.c
+++ b/drivers/tty/serial/sc16is7xx.c
@@ -1342,9 +1342,18 @@ static int sc16is7xx_gpio_direction_output(struct gpio_chip *chip,
                state |= BIT(offset);
        else
                state &= ~BIT(offset);
-       sc16is7xx_port_write(port, SC16IS7XX_IOSTATE_REG, state);
+
+       /*
+        * If we write IOSTATE first, and then IODIR, the output value is not
+        * transferred to the corresponding I/O pin.
+        * The datasheet states that each register bit will be transferred to
+        * the corresponding I/O pin programmed as output when writing to
+        * IOSTATE. Therefore, configure direction first with IODIR, and then
+        * set value after with IOSTATE.
+        */
        sc16is7xx_port_update(port, SC16IS7XX_IODIR_REG, BIT(offset),
                              BIT(offset));
+       sc16is7xx_port_write(port, SC16IS7XX_IOSTATE_REG, state);
 
        return 0;
 }