Hi Sherry,
thanks for re-spin!
On Tue, 2024-03-05 at 09:57 +0800, Sherry Sun wrote:
> If the remote uart device is not connected or not enabled after booting
> up, the CTS line is high by default. At this time, if we enable the flow
> control when opening the device(for example, using “stty -F /dev/ttyLP4
> crtscts” command), there will be a pending idle preamble(first writing 0
> and then writing 1 to UARTCTRL_TE will queue an idle preamble) that
> cannot be sent out, resulting in the uart port fail to close(waiting for
> TX empty), so the user space stty will have to wait for a long time or
> forever.
>
> This is an LPUART IP bug(idle preamble has higher priority than CTS),
> here add a workaround patch to enable TX CTS after enabling UARTCTRL_TE,
> so that the idle preamble does not get stuck due to CTS is deasserted.
>
> Fixes: 380c966c093e ("tty: serial: fsl_lpuart: add 32-bit register interface support")
> Signed-off-by: Sherry Sun <sherry.sun@xxxxxxx>
Reviewed-by: Alexander Sverdlin <alexander.sverdlin@xxxxxxxxxxx>
> ---
> Changes in V2:
> 1. Move the "restore control register" comment message to the appropriate place.
> 2. Add Fixes tag.
> ---
> drivers/tty/serial/fsl_lpuart.c | 7 +++++--
> 1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/tty/serial/fsl_lpuart.c b/drivers/tty/serial/fsl_lpuart.c
> index 5ddf110aedbe..bbcbc91482af 100644
> --- a/drivers/tty/serial/fsl_lpuart.c
> +++ b/drivers/tty/serial/fsl_lpuart.c
> @@ -2345,9 +2345,12 @@ lpuart32_set_termios(struct uart_port *port, struct ktermios *termios,
>
> lpuart32_write(&sport->port, bd, UARTBAUD);
> lpuart32_serial_setbrg(sport, baud);
> - lpuart32_write(&sport->port, modem, UARTMODIR);
> - lpuart32_write(&sport->port, ctrl, UARTCTRL);
> + /* disable CTS before enabling UARTCTRL_TE to avoid pending idle preamble */
> + lpuart32_write(&sport->port, modem & ~UARTMODIR_TXCTSE, UARTMODIR);
> /* restore control register */
> + lpuart32_write(&sport->port, ctrl, UARTCTRL);
> + /* re-enable the CTS if needed */
> + lpuart32_write(&sport->port, modem, UARTMODIR);
>
> if ((ctrl & (UARTCTRL_PE | UARTCTRL_M)) == UARTCTRL_PE)
> sport->is_cs7 = true;
--
Alexander Sverdlin
Siemens AG
www.siemens.com
