[PATCH v7 5/9] serial: max310x: stop the transmitter before powering down in shutdown

[Tapio Reijonen]

max310x_shutdown() powers the port down regardless of what the
transmitter is doing. The power-off stops the UART clock mid-character,
truncating the final frame, and on the auto-RTS path it freezes the
RTS output at its current level: a close() with data still queued
leaves the RS485 transceiver asserted until the next open, which also
emits the interrupted character corrupted.

Waiting for the data to drain is no better: the tty layer's
wait-until-sent is bounded by closing_wait, configurable to none, and
a hangup arrives with no wait at all, so draining a full FIFO blocks
close() in uninterruptible sleep for seconds at low baud rates - and
forever when CTS flow control blocks the FIFO.

Instead, set MODE1 TxDisabl: the character in flight completes and the
transmitter ceases with TX_ at idle. Give that character one character
time (the chip has no transmitter-idle status), then reset the FIFOs
so the auto-RTS engine sees the transmitter empty and releases RTS
within the configured after-send hold; wait that hold plus one bit
time before powering off. Abandoned data was explicitly not waited
for, and startup() resets the FIFOs and TxDisabl on open anyway.

Measured on a MAX14830: a truncating close() at 50 baud takes 0.23 s,
RTS releases one character plus the hold after the last stop bit on
both polarities, and the reopen corruption is gone.

Fixes: f65444187a66 ("serial: New serial driver MAX310X")
Signed-off-by: Tapio Reijonen <tapio.reijonen@xxxxxxxxxxx>
---
 drivers/tty/serial/max310x.c | 44 ++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 40 insertions(+), 4 deletions(-)

diff --git a/drivers/tty/serial/max310x.c b/drivers/tty/serial/max310x.c
index 2456a2af891f5296cfec833d6406916073d2865f..e08e506360d72a3ef4c0ebdca2e60d864973f909 100644
--- a/drivers/tty/serial/max310x.c
+++ b/drivers/tty/serial/max310x.c
@@ -302,6 +302,7 @@ struct max310x_one {
 	struct work_struct	md_work;
 	struct work_struct	rs_work;
 	struct regmap		*regmap;
+	unsigned int		char_time_us;
 	unsigned int		baud;
 	bool			tx_break;	/* break_ctl() owns the transceiver */
 
@@ -1012,6 +1013,7 @@ static void max310x_set_termios(struct uart_port *port,
 				struct ktermios *termios,
 				const struct ktermios *old)
 {
+	unsigned int frame_bits = tty_get_frame_size(termios->c_cflag);
 	unsigned int lcr = 0, flow = 0;
 	int baud;
 
@@ -1124,10 +1126,13 @@ static void max310x_set_termios(struct uart_port *port,
 	uart_update_timeout(port, termios->c_cflag, baud);
 
 	/*
-	 * Cache the new baud rate and reprogram the RS485 RTS delays, whose
-	 * millisecond-to-bit-time conversion depends on it.
+	 * Cache the new baud rate and the time it takes to clock out one
+	 * character before reprogramming the RS485 RTS delays: the
+	 * millisecond-to-bit-time conversion divides by the baud rate.
 	 */
 	to_max310x_port(port)->baud = baud;
+	to_max310x_port(port)->char_time_us =
+		DIV_ROUND_UP(USEC_PER_SEC * frame_bits, baud);
 	max310x_set_rts_ctl_params(to_max310x_port(port));
 }
 
@@ -1184,9 +1189,10 @@ static int max310x_startup(struct uart_port *port)
 	one->tx_break = false;
 	max310x_port_update(port, MAX310X_LCR_REG, MAX310X_LCR_TXBREAK_BIT, 0);
 
-	/* Configure MODE1 register */
+	/* Configure MODE1: re-enable the transmitter shutdown() stopped */
 	max310x_port_update(port, MAX310X_MODE1_REG,
-			    MAX310X_MODE1_TRNSCVCTRL_BIT, 0);
+			    MAX310X_MODE1_TRNSCVCTRL_BIT |
+			    MAX310X_MODE1_TXDIS_BIT, 0);
 
 	/* Configure MODE2 register & Reset FIFOs*/
 	val = MAX310X_MODE2_RXEMPTINV_BIT | MAX310X_MODE2_FIFORST_BIT;
@@ -1223,9 +1229,39 @@ static int max310x_startup(struct uart_port *port)
 
 static void max310x_shutdown(struct uart_port *port)
 {
+	struct max310x_one *one = to_max310x_port(port);
+
 	/* Disable all interrupts */
 	max310x_port_write(port, MAX310X_IRQEN_REG, 0);
 
+	/*
+	 * Stop the transmitter: the character in flight completes, data
+	 * still queued is abandoned (the next startup() resets the FIFOs).
+	 */
+	max310x_port_update(port, MAX310X_MODE1_REG,
+			    MAX310X_MODE1_TXDIS_BIT, MAX310X_MODE1_TXDIS_BIT);
+
+	/*
+	 * Let the character in flight finish (the chip has no
+	 * transmitter-idle status), then empty the FIFO: auto-RTS releases
+	 * only once the transmitter is empty, and the power-off below
+	 * would freeze an asserted pin until the next open.
+	 */
+	fsleep(one->char_time_us);
+	max310x_port_update(port, MAX310X_MODE2_REG,
+			    MAX310X_MODE2_FIFORST_BIT,
+			    MAX310X_MODE2_FIFORST_BIT);
+	max310x_port_update(port, MAX310X_MODE2_REG,
+			    MAX310X_MODE2_FIFORST_BIT, 0);
+	if (port->rs485.flags & SER_RS485_ENABLED && one->baud) {
+		unsigned int hold = DIV_ROUND_UP(one->baud *
+				port->rs485.delay_rts_after_send,
+				MSEC_PER_SEC);
+
+		fsleep(max(DIV_ROUND_UP((hold + 1) * USEC_PER_SEC,
+					one->baud), 100U));
+	}
+
 	max310x_power(port, 0);
 }
 

-- 
2.47.3