linux-kernel - Re: [PATCH V2] tty: serial: fsl_lpuart: avoid idle preamble pending if CTS is enabled

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Message-ID: <317ddcba7d49ab22c3a138c34235a1118f9c9297.camel@siemens.com>
Date: Tue, 5 Mar 2024 07:26:47 +0000
From: "Sverdlin, Alexander" <alexander.sverdlin@...mens.com>
To: "u.kleine-koenig@...gutronix.de" <u.kleine-koenig@...gutronix.de>,
	"sherry.sun@....com" <sherry.sun@....com>, "gregkh@...uxfoundation.org"
	<gregkh@...uxfoundation.org>, "ilpo.jarvinen@...ux.intel.com"
	<ilpo.jarvinen@...ux.intel.com>, "robh@...nel.org" <robh@...nel.org>,
	"shenwei.wang@....com" <shenwei.wang@....com>, "tglx@...utronix.de"
	<tglx@...utronix.de>, "jirislaby@...nel.org" <jirislaby@...nel.org>,
	"robert.hodaszi@...i.com" <robert.hodaszi@...i.com>
CC: "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
	"imx@...ts.linux.dev" <imx@...ts.linux.dev>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "frank.li@....com" <frank.li@....com>
Subject: Re: [PATCH V2] tty: serial: fsl_lpuart: avoid idle preamble pending
 if CTS is enabled

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@....com>

Reviewed-by: Alexander Sverdlin <alexander.sverdlin@...mens.com>

> ---
> 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