serial: stm32: Use TC interrupt to deassert GPIO RTS in RS485 mode

In case the RS485 mode is emulated using GPIO RTS, use the TC interrupt
to deassert the GPIO RTS, otherwise the GPIO RTS stays asserted after a
transmission ended and the RS485 cannot work.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Alexandre Torgue <alexandre.torgue@foss.st.com>
Cc: Erwan Le Ray <erwan.leray@foss.st.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jean Philippe Romain <jean-philippe.romain@foss.st.com>
Cc: Valentin Caron <valentin.caron@foss.st.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-stm32@st-md-mailman.stormreply.com
To: linux-serial@vger.kernel.org
Link: https://lore.kernel.org/r/20220430162845.244655-2-marex@denx.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/stm32-usart.c

@@ -417,6 +417,14 @@ static void stm32_usart_tx_interrupt_enable(struct uart_port *port)
 		stm32_usart_set_bits(port, ofs->cr1, USART_CR1_TXEIE);
 }
 
+static void stm32_usart_tc_interrupt_enable(struct uart_port *port)
+{
+	struct stm32_port *stm32_port = to_stm32_port(port);
+	const struct stm32_usart_offsets *ofs = &stm32_port->info->ofs;
+
+	stm32_usart_set_bits(port, ofs->cr1, USART_CR1_TCIE);
+}
+
 static void stm32_usart_rx_dma_complete(void *arg)
 {
 	struct uart_port *port = arg;
@@ -442,6 +450,14 @@ static void stm32_usart_tx_interrupt_disable(struct uart_port *port)
 		stm32_usart_clr_bits(port, ofs->cr1, USART_CR1_TXEIE);
 }
 
+static void stm32_usart_tc_interrupt_disable(struct uart_port *port)
+{
+	struct stm32_port *stm32_port = to_stm32_port(port);
+	const struct stm32_usart_offsets *ofs = &stm32_port->info->ofs;
+
+	stm32_usart_clr_bits(port, ofs->cr1, USART_CR1_TCIE);
+}
+
 static void stm32_usart_rs485_rts_enable(struct uart_port *port)
 {
 	struct stm32_port *stm32_port = to_stm32_port(port);
@@ -585,6 +601,13 @@ static void stm32_usart_transmit_chars(struct uart_port *port)
 	u32 isr;
 	int ret;
 
+	if (!stm32_port->hw_flow_control &&
+	    port->rs485.flags & SER_RS485_ENABLED) {
+		stm32_port->txdone = false;
+		stm32_usart_tc_interrupt_disable(port);
+		stm32_usart_rs485_rts_enable(port);
+	}
+
 	if (port->x_char) {
 		if (stm32_usart_tx_dma_started(stm32_port) &&
 		    stm32_usart_tx_dma_enabled(stm32_port))
@@ -625,8 +648,14 @@ static void stm32_usart_transmit_chars(struct uart_port *port)
 	if (uart_circ_chars_pending(xmit) < WAKEUP_CHARS)
 		uart_write_wakeup(port);
 
-	if (uart_circ_empty(xmit))
+	if (uart_circ_empty(xmit)) {
 		stm32_usart_tx_interrupt_disable(port);
+		if (!stm32_port->hw_flow_control &&
+		    port->rs485.flags & SER_RS485_ENABLED) {
+			stm32_port->txdone = true;
+			stm32_usart_tc_interrupt_enable(port);
+		}
+	}
 }
 
 static irqreturn_t stm32_usart_interrupt(int irq, void *ptr)
@@ -640,6 +669,13 @@ static irqreturn_t stm32_usart_interrupt(int irq, void *ptr)
 
 	sr = readl_relaxed(port->membase + ofs->isr);
 
+	if (!stm32_port->hw_flow_control &&
+	    port->rs485.flags & SER_RS485_ENABLED &&
+	    (sr & USART_SR_TC)) {
+		stm32_usart_tc_interrupt_disable(port);
+		stm32_usart_rs485_rts_disable(port);
+	}
+
 	if ((sr & USART_SR_RTOF) && ofs->icr != UNDEF_REG)
 		writel_relaxed(USART_ICR_RTOCF,
 			       port->membase + ofs->icr);
@@ -763,8 +799,10 @@ static void stm32_usart_start_tx(struct uart_port *port)
 {
 	struct circ_buf *xmit = &port->state->xmit;
 
-	if (uart_circ_empty(xmit) && !port->x_char)
+	if (uart_circ_empty(xmit) && !port->x_char) {
+		stm32_usart_rs485_rts_disable(port);
 		return;
+	}
 
 	stm32_usart_rs485_rts_enable(port);
 

drivers/tty/serial/stm32-usart.h

@@ -271,6 +271,7 @@ struct stm32_port {
 	bool hw_flow_control;
 	bool swap;		 /* swap RX & TX pins */
 	bool fifoen;
+	bool txdone;
 	int rxftcfg;		/* RX FIFO threshold CFG      */
 	int txftcfg;		/* TX FIFO threshold CFG      */
 	bool wakeup_src;