linux-kernel - [PATCH v2] serial:xilinx_uartps:fix rs485 delay_rts_after

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Message-Id: <20251221103221.1971125-1-jakub.turek@elsta.tech>
Date: Sun, 21 Dec 2025 11:32:21 +0100
From: "j.turek" <jakub.turek@...ta.tech>
To: gregkh@...uxfoundation.org,
	jirislaby@...nel.org
Cc: michal.simek@....com,
	namcao@...utronix.de,
	tglx@...utronix.de,
	zack.rusin@...adcom.com,
	sean.anderson@...ux.dev,
	hshah@...ado.com,
	linux-serial@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org,
	"j.turek" <jakub.turek@...ta.tech>
Subject: [PATCH v2] serial:xilinx_uartps:fix rs485 delay_rts_after_send

RTS line control with delay should be triggered when there is no more bytes
in kfifo and hardware buffer is empty. Without this patch RTS control is
scheduled right after feeding hardware buffer and this is too early.
RTS line may change state before hardware buffer is empty.
With this patch delayed RTS state change is triggered when function
cdns_uart_handle_tx is called from cdns_uart_isr on CDNS_UART_IXR_TXEMPTY
exactly when hardware completed transmission

Signed-off-by: Jakub Turek  <jakub.turek@...ta.tech>

Fixes: fccc9d9233f9 ("tty: serial: uartps: Add rs485 support to uartps driver")
---
 drivers/tty/serial/xilinx_uartps.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

v2:
 - Fix build error by declaring rts_delay as ktime_t

diff --git a/drivers/tty/serial/xilinx_uartps.c b/drivers/tty/serial/xilinx_uartps.c
index 1e4d54fd5762..8b08f478253e 100644
--- a/drivers/tty/serial/xilinx_uartps.c
+++ b/drivers/tty/serial/xilinx_uartps.c
@@ -428,10 +428,17 @@ static void cdns_uart_handle_tx(void *dev_id)
 	struct tty_port *tport = &port->state->port;
 	unsigned int numbytes;
 	unsigned char ch;
+	ktime_t rts_delay;
 
 	if (kfifo_is_empty(&tport->xmit_fifo) || uart_tx_stopped(port)) {
 		/* Disable the TX Empty interrupt */
 		writel(CDNS_UART_IXR_TXEMPTY, port->membase + CDNS_UART_IDR);
+		/* Set RTS line after delay */
+		if (cdns_uart->port->rs485.flags & SER_RS485_ENABLED) {
+			cdns_uart->tx_timer.function = &cdns_rs485_rx_callback;
+			rts_delay = ns_to_ktime(cdns_calc_after_tx_delay(cdns_uart));
+			hrtimer_start(&cdns_uart->tx_timer, rts_delay, HRTIMER_MODE_REL);
+		}
 		return;
 	}
 
@@ -448,13 +455,6 @@ static void cdns_uart_handle_tx(void *dev_id)
 
 	/* Enable the TX Empty interrupt */
 	writel(CDNS_UART_IXR_TXEMPTY, cdns_uart->port->membase + CDNS_UART_IER);
-
-	if (cdns_uart->port->rs485.flags & SER_RS485_ENABLED &&
-	    (kfifo_is_empty(&tport->xmit_fifo) || uart_tx_stopped(port))) {
-		hrtimer_update_function(&cdns_uart->tx_timer, cdns_rs485_rx_callback);
-		hrtimer_start(&cdns_uart->tx_timer,
-			      ns_to_ktime(cdns_calc_after_tx_delay(cdns_uart)), HRTIMER_MODE_REL);
-	}
 }
 
 /**
-- 
2.34.1