Am 25.08.25 um 13:58 schrieb Maarten Brock:

Hello Martin,

Hi Maarten,

Why not just start the timer and check TEMT after it has elapsed and restart the timer if not empty? It would prevent busy-loop waiting.

It would, yes, but couldn't this cause the time between last transmitted byte and switching the RTS GPIO to be less than the specified RTS delay?

Martin

Kind regards, Maarten Brock

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From: Martin Kaistra martin.kaistra@linutronix.de Sent: Monday, August 25, 2025 11:22 To: Michal Simek michal.simek@amd.com; Greg Kroah-Hartman gregkh@linuxfoundation.org; linux-serial@vger.kernel.org linux-serial@vger.kernel.org Cc: Manikanta Guntupalli manikanta.guntupalli@amd.com; stable@vger.kernel.org stable@vger.kernel.org Subject: [PATCH v2] serial: uartps: do not deassert RS485 RTS GPIO prematurely

After all bytes to be transmitted have been written to the FIFO register, the hardware might still be busy actually sending them.

Thus, wait for the TX FIFO to be empty before starting the timer for the RTS after send delay.

Cc: stable@vger.kernel.org Fixes: fccc9d9233f9 ("tty: serial: uartps: Add rs485 support to uartps driver") Signed-off-by: Martin Kaistra martin.kaistra@linutronix.de

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Changes in v2:

• Add cc stable
• Add timeout

drivers/tty/serial/xilinx_uartps.c | 9 ++++++++-  1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/serial/xilinx_uartps.c b/drivers/tty/serial/xilinx_uartps.c index fe457bf1e15bb..e1dd3843d563c 100644 --- a/drivers/tty/serial/xilinx_uartps.c +++ b/drivers/tty/serial/xilinx_uartps.c @@ -429,7 +429,7 @@ static void cdns_uart_handle_tx(void *dev_id)          struct uart_port *port = (struct uart_port *)dev_id;          struct cdns_uart *cdns_uart = port->private_data;          struct tty_port *tport = &port->state->port; -       unsigned int numbytes; +       unsigned int numbytes, tmout;          unsigned char ch;          if (kfifo_is_empty(&tport->xmit_fifo) || uart_tx_stopped(port)) { @@ -454,6 +454,13 @@ static void cdns_uart_handle_tx(void *dev_id)          if (cdns_uart->port->rs485.flags & SER_RS485_ENABLED &&              (kfifo_is_empty(&tport->xmit_fifo) || uart_tx_stopped(port))) { +               /* Wait for the tx fifo to be actually empty */ +               for (tmout = 1000000; tmout; tmout--) { +                       if (cdns_uart_tx_empty(port) == TIOCSER_TEMT) +                               break; +                       udelay(1); +               }

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);