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

Fixes: fccc9d9 ("tty: serial: uartps: Add rs485 support to uartps driver")
Cc: stable <stable@kernel.org>
Link: https://patch.msgid.link/20251221103221.1971125-1-jakub.turek@elsta.tech
Signed-off-by: Jakub Turek  <jakub.turek@elsta.tech>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: j.turek

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);
-	}
 }
 
 /**