[PATCH v6 6/9] serial: max310x: support active-low RTS on the hardware path

[Tapio Reijonen]

The chip's auto-RTS engine asserts the RTS_ pin high while data is
shifting out, so a transceiver with an active-low driver-enable could
not use the hardware RS485 path at all: SER_RS485_RTS_AFTER_SEND is
not in the supported flags and the core normalizes it away with
"invalid RTS setting, using RTS_ON_SEND instead".

The output stage is invertible: program IRDA.RTSINVERT when the
requested polarity is active-low and advertise SER_RS485_RTS_AFTER_SEND
in rs485_supported. A break already drives break_state onto the RTS
bit unadjusted, which remains correct because RTSINVERT inverts the
output stage itself, not the register value.

Assisted-by: Claude:claude-fable-5
Signed-off-by: Tapio Reijonen <tapio.reijonen@xxxxxxxxxxx>
---
 drivers/tty/serial/max310x.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/drivers/tty/serial/max310x.c b/drivers/tty/serial/max310x.c
index 9dcc817b9794537385aa9ca726cb1cc1b2af6b4c..810b749bfe9ad9b4d75a7b158ed8013a5a4f2078 100644
--- a/drivers/tty/serial/max310x.c
+++ b/drivers/tty/serial/max310x.c
@@ -164,6 +164,7 @@
 /* IRDA register bits */
 #define MAX310X_IRDA_IRDAEN_BIT		(1 << 0) /* IRDA mode enable */
 #define MAX310X_IRDA_SIR_BIT		(1 << 1) /* SIR mode enable */
+#define MAX310X_IRDA_RTSINVERT_BIT	(1 << 2) /* Invert RTS output */
 
 /* HDPIXDELAY accessor macros */
 #define MAX310X_HDPIXDELAY_SETUP(val)	(((val) & 0x0f) << 4)
@@ -951,7 +952,7 @@ static void max310x_set_rts_ctl_params(struct max310x_one *one)
 	const unsigned int max_bit_dly = 15;
 	struct uart_port *port = &one->port;
 	unsigned int setup = 0, hold = 0;
-	u8 mode1 = 0;
+	u8 mode1 = 0, irda = 0;
 
 	if (port->rs485.flags & SER_RS485_ENABLED) {
 		/* Convert milliseconds to bit-times, rounding up. */
@@ -963,6 +964,12 @@ static void max310x_set_rts_ctl_params(struct max310x_one *one)
 		hold  = min(hold,  max_bit_dly);
 
 		mode1 = MAX310X_MODE1_TRNSCVCTRL_BIT;
+		/*
+		 * The auto-RTS engine asserts RTS high on send; for an
+		 * active-low RTS let IRDA.RTSINVERT invert the output stage.
+		 */
+		if (!(port->rs485.flags & SER_RS485_RTS_ON_SEND))
+			irda = MAX310X_IRDA_RTSINVERT_BIT;
 	}
 
 	max310x_port_write(port, MAX310X_HDPIXDELAY_REG,
@@ -980,6 +987,8 @@ static void max310x_set_rts_ctl_params(struct max310x_one *one)
 
 	max310x_port_update(port, MAX310X_MODE1_REG,
 			    MAX310X_MODE1_TRNSCVCTRL_BIT, mode1);
+	max310x_port_update(port, MAX310X_IRDA_REG,
+			    MAX310X_IRDA_RTSINVERT_BIT, irda);
 }
 
 static void max310x_break_ctl(struct uart_port *port, int break_state)
@@ -996,7 +1005,9 @@ static void max310x_break_ctl(struct uart_port *port, int break_state)
 	 * The chip's auto-RTS asserts the transceiver only while FIFO data is
 	 * shifting out, and a break is not FIFO data. Disable auto-RTS for the
 	 * break duration and drive RTS manually so the break reaches the wire;
-	 * restore auto-RTS when the break ends.
+	 * restore auto-RTS when the break ends. For an active-low RTS,
+	 * IRDA.RTSINVERT already inverts the RTS_ output stage, so break_state
+	 * is driven as it is.
 	 */
 	if (break_state) {
 		if (!(port->rs485.flags & SER_RS485_ENABLED))
@@ -1401,7 +1412,8 @@ static int max310x_gpio_set_config(struct gpio_chip *chip, unsigned int offset,
 #endif
 
 static const struct serial_rs485 max310x_rs485_supported = {
-	.flags = SER_RS485_ENABLED | SER_RS485_RTS_ON_SEND | SER_RS485_RX_DURING_TX,
+	.flags = SER_RS485_ENABLED | SER_RS485_RTS_ON_SEND |
+		 SER_RS485_RTS_AFTER_SEND | SER_RS485_RX_DURING_TX,
 	.delay_rts_before_send = 1,
 	.delay_rts_after_send = 1,
 };

-- 
2.47.3