[PATCH] serial: amba-pl011: honour delay_rts_before_send for every write

[PATCH] serial: amba-pl011: honour delay_rts_before_send for every write

[Nicolai Buchwitz]

While the driver waits out delay_rts_before_send, another write calls
pl011_start_tx() again. pl011_rs485_tx_start() then switches straight
to SEND, and the data goes out before the delay has passed.

Leave the start to the trigger_start_tx timer while it is pending.

Fixes: 2c1fd53af21b ("serial: amba-pl011: Fix RTS handling in RS485 mode")
Cc: stable@vger.kernel.org
Signed-off-by: Nicolai Buchwitz <nb@tipi-net.de>
---
Noticed while implementing a DMX master on a CM5 with its PL011.
Tested on that board with delay_rts_before_send set to 1 ms and two
quick writes. With ftrace, the first pl011_tx_chars() came 0.03 ms
after pl011_start_tx() before this patch and 1.03 ms after with it.
---
 drivers/tty/serial/amba-pl011.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c
index c4824c201e1c..759124a2a1a8 100644
--- a/drivers/tty/serial/amba-pl011.c
+++ b/drivers/tty/serial/amba-pl011.c
@@ -1420,10 +1420,6 @@ static void pl011_rs485_tx_start(struct uart_amba_port *uap)
 	struct uart_port *port = &uap->port;
 	u32 cr;
 
-	if (uap->rs485_tx_state == WAIT_AFTER_RTS) {
-		uap->rs485_tx_state = SEND;
-		return;
-	}
 	if (uap->rs485_tx_state == WAIT_AFTER_SEND ||
 	    uap->rs485_tx_state == WAIT_AFTER_SEND_DELAY) {
 		hrtimer_try_to_cancel(&uap->trigger_stop_tx);
@@ -1462,6 +1458,10 @@ static void pl011_start_tx(struct uart_port *port)
 
 	if ((uap->port.rs485.flags & SER_RS485_ENABLED) &&
 	    uap->rs485_tx_state != SEND) {
+		/* trigger_start_tx starts TX once delay_rts_before_send passed */
+		if (uap->rs485_tx_state == WAIT_AFTER_RTS)
+			return;
+
 		pl011_rs485_tx_start(uap);
 		if (uap->rs485_tx_state == WAIT_AFTER_RTS)
 			return;
@@ -1478,8 +1478,10 @@ static enum hrtimer_restart pl011_trigger_start_tx(struct hrtimer *t)
 	unsigned long flags;
 
 	uart_port_lock_irqsave(&uap->port, &flags);
-	if (uap->rs485_tx_state == WAIT_AFTER_RTS)
+	if (uap->rs485_tx_state == WAIT_AFTER_RTS) {
+		uap->rs485_tx_state = SEND;
 		pl011_start_tx(&uap->port);
+	}
 	uart_port_unlock_irqrestore(&uap->port, flags);
 
 	return HRTIMER_NORESTART;

---
base-commit: a90ee4305c4a5df72c11b31dacfdc76e00fcf78a
change-id: 20261011-fix-pl011-rs485-rts-delay-cad4edad250b

Best regards,
--  
Nicolai Buchwitz <nb@tipi-net.de>

Re: [PATCH] serial: amba-pl011: honour delay_rts_before_send for every write

[sashiko-bot]

> While the driver waits out delay_rts_before_send, another write calls
> pl011_start_tx() again. pl011_rs485_tx_start() then switches straight
> to SEND, and the data goes out before the delay has passed.
> 
> Leave the start to the trigger_start_tx timer while it is pending.
> 
> Fixes: 2c1fd53af21b ("serial: amba-pl011: Fix RTS handling in RS485 mode")
> Cc: stable@vger.kernel.org
> Signed-off-by: Nicolai Buchwitz <nb@tipi-net.de>

Sashiko has reviewed this patch and found no issues. It looks great!

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261011-fix-pl011-rs485-rts-delay-v1-1-5078e61bc408@tipi-net.de?part=1