serial: omap: fix rs485 half-duplex filtering

Data received during half-duplex transmission must be filtered.
If the target device responds quickly, emptying the FIFO at the end of
the transmission can erase not only the echo characters but also part of
the response message.
By keeping the receive interrupt enabled even during transmission, it
allows you to filter each echo character and only in a number equal to
those transmitted.
The issue was generated by a target device that started responding
240us later having received a request in communication at 115200bps.
Sometimes, some messages received by the target were missing some of the
first bytes.

Fixes: 3a13884abea0 ("tty/serial: omap: empty the RX FIFO at the end of half-duplex TX")
Signed-off-by: Dario Binacchi <dariobin@libero.it>
Link: https://lore.kernel.org/r/20210418094705.27014-1-dariobin@libero.it
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 24 insertions, 15 deletions

diff --git a/drivers/tty/serial/omap-serial.c b/drivers/tty/serial/omap-serial.c
index 1583e93b2202..84e8158088cd 100644
--- a/drivers/tty/serial/omap-serial.c
+++ b/drivers/tty/serial/omap-serial.c
@@ -159,6 +159,8 @@ struct uart_omap_port {
 	u32			calc_latency;
 	struct work_struct	qos_work;
 	bool			is_suspending;
+
+	unsigned int		rs485_tx_filter_count;
 };
 
 #define to_uart_omap_port(p) ((container_of((p), struct uart_omap_port, port)))
@@ -329,19 +331,6 @@ static void serial_omap_stop_tx(struct uart_port *port)
 		serial_out(up, UART_IER, up->ier);
 	}
 
-	if ((port->rs485.flags & SER_RS485_ENABLED) &&
-	    !(port->rs485.flags & SER_RS485_RX_DURING_TX)) {
-		/*
-		 * Empty the RX FIFO, we are not interested in anything
-		 * received during the half-duplex transmission.
-		 */
-		serial_out(up, UART_FCR, up->fcr | UART_FCR_CLEAR_RCVR);
-		/* Re-enable RX interrupts */
-		up->ier |= UART_IER_RLSI | UART_IER_RDI;
-		up->port.read_status_mask |= UART_LSR_DR;
-		serial_out(up, UART_IER, up->ier);
-	}
-
 	pm_runtime_mark_last_busy(up->dev);
 	pm_runtime_put_autosuspend(up->dev);
 }
@@ -367,6 +356,10 @@ static void transmit_chars(struct uart_omap_port *up, unsigned int lsr)
 		serial_out(up, UART_TX, up->port.x_char);
 		up->port.icount.tx++;
 		up->port.x_char = 0;
+		if ((up->port.rs485.flags & SER_RS485_ENABLED) &&
+		    !(up->port.rs485.flags & SER_RS485_RX_DURING_TX))
+			up->rs485_tx_filter_count++;
+
 		return;
 	}
 	if (uart_circ_empty(xmit) || uart_tx_stopped(&up->port)) {
@@ -378,6 +371,10 @@ static void transmit_chars(struct uart_omap_port *up, unsigned int lsr)
 		serial_out(up, UART_TX, xmit->buf[xmit->tail]);
 		xmit->tail = (xmit->tail + 1) & (UART_XMIT_SIZE - 1);
 		up->port.icount.tx++;
+		if ((up->port.rs485.flags & SER_RS485_ENABLED) &&
+		    !(up->port.rs485.flags & SER_RS485_RX_DURING_TX))
+			up->rs485_tx_filter_count++;
+
 		if (uart_circ_empty(xmit))
 			break;
 	} while (--count > 0);
@@ -421,7 +418,7 @@ static void serial_omap_start_tx(struct uart_port *port)
 
 	if ((port->rs485.flags & SER_RS485_ENABLED) &&
 	    !(port->rs485.flags & SER_RS485_RX_DURING_TX))
-		serial_omap_stop_rx(port);
+		up->rs485_tx_filter_count = 0;
 
 	serial_omap_enable_ier_thri(up);
 	pm_runtime_mark_last_busy(up->dev);
@@ -492,8 +489,13 @@ static void serial_omap_rlsi(struct uart_omap_port *up, unsigned int lsr)
 	 * Read one data character out to avoid stalling the receiver according
 	 * to the table 23-246 of the omap4 TRM.
 	 */
-	if (likely(lsr & UART_LSR_DR))
+	if (likely(lsr & UART_LSR_DR)) {
 		serial_in(up, UART_RX);
+		if ((up->port.rs485.flags & SER_RS485_ENABLED) &&
+		    !(up->port.rs485.flags & SER_RS485_RX_DURING_TX) &&
+		    up->rs485_tx_filter_count)
+			up->rs485_tx_filter_count--;
+	}
 
 	up->port.icount.rx++;
 	flag = TTY_NORMAL;
@@ -544,6 +546,13 @@ static void serial_omap_rdi(struct uart_omap_port *up, unsigned int lsr)
 		return;
 
 	ch = serial_in(up, UART_RX);
+	if ((up->port.rs485.flags & SER_RS485_ENABLED) &&
+	    !(up->port.rs485.flags & SER_RS485_RX_DURING_TX) &&
+	    up->rs485_tx_filter_count) {
+		up->rs485_tx_filter_count--;
+		return;
+	}
+
 	flag = TTY_NORMAL;
 	up->port.icount.rx++;