[PATCH 2/2] serial: stm32: Use TC interrupt to deassert GPIO RTS in RS485 mode

[Marek Vasut <marex@denx.de>]

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
---
 drivers/tty/serial/stm32-usart.c | 42 ++++++++++++++++++++++++++++++--
 drivers/tty/serial/stm32-usart.h |  1 +
 2 files changed, 41 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/stm32-usart.c b/drivers/tty/serial/stm32-usart.c
index 224f359c6051e..764415b8e8f03 100644
--- a/drivers/tty/serial/stm32-usart.c
+++ b/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);
 
diff --git a/drivers/tty/serial/stm32-usart.h b/drivers/tty/serial/stm32-usart.h
index d734c4a5fd24c..ee69c203b926d 100644
--- a/drivers/tty/serial/stm32-usart.h
+++ b/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;
-- 
2.35.1