linux-kernel - Re: [PATCH] tty: serial: fsl_lpuart: don't break the on-going transfer when global reset

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Date:   Wed, 19 Oct 2022 12:19:46 +0300 (EEST)
From:   Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To:     Sherry Sun <sherry.sun@....com>
cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Lukas Wunner <lukas@...ner.de>,
        linux-serial <linux-serial@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-imx@....com
Subject: Re: [PATCH] tty: serial: fsl_lpuart: don't break the on-going transfer
 when global reset

On Wed, 19 Oct 2022, Sherry Sun wrote:

> lpuart_global_reset() shouldn't break the on-going transmit engin, need
> to recover the on-going data transfer after reset.
> 
> This can help earlycon here, since commit 60f361722ad2 ("serial:
> fsl_lpuart: Reset prior to registration") moved lpuart_global_reset()
> before uart_add_one_port(), earlycon is writing during global reset,
> as global reset will disable the TX and clear the baud rate register,
> which caused the earlycon cannot work any more after reset, needs to
> restore the baud rate and re-enable the transmitter to recover the
> earlycon write.
>
> Fixes: bd5305dcabbc ("tty: serial: fsl_lpuart: do software reset for imx7ulp and imx8qxp")
> Signed-off-by: Sherry Sun <sherry.sun@....com>
> ---
>  drivers/tty/serial/fsl_lpuart.c | 23 ++++++++++++++++++++---
>  1 file changed, 20 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/tty/serial/fsl_lpuart.c b/drivers/tty/serial/fsl_lpuart.c
> index 67fa113f77d4..5064fdba1b61 100644
> --- a/drivers/tty/serial/fsl_lpuart.c
> +++ b/drivers/tty/serial/fsl_lpuart.c
> @@ -408,11 +408,9 @@ static int lpuart_global_reset(struct lpuart_port *sport)
>  {
>  	struct uart_port *port = &sport->port;
>  	void __iomem *global_addr;
> +	unsigned long tx_enable, bd, stat, sfifo;
>  	int ret;
>  
> -	if (uart_console(port))
> -		return 0;
> -
>  	ret = clk_prepare_enable(sport->ipg_clk);
>  	if (ret) {
>  		dev_err(sport->port.dev, "failed to enable uart ipg clk: %d\n", ret);
> @@ -420,11 +418,30 @@ static int lpuart_global_reset(struct lpuart_port *sport)
>  	}
>  
>  	if (is_imx7ulp_lpuart(sport) || is_imx8qxp_lpuart(sport)) {
> +		/*
> +		 * If the transmitter is used by earlycon, wait transmit engin complete
> +		 * and then reset
> +		 */
> +		tx_enable = lpuart32_read(port, UARTCTRL) & UARTCTRL_TE;
> +		if (tx_enable) {
> +			bd = lpuart32_read(&sport->port, UARTBAUD);
> +			stat = lpuart32_read(port, UARTSTAT);
> +			sfifo = lpuart32_read(port, UARTFIFO);
> +			while (!(stat & UARTSTAT_TC && sfifo & UARTFIFO_TXEMPT))
> +				cpu_relax();

This loop, if ever taken once, will loop forever as neither stat nor sfifo 
are reread inside the loop.

-- 
 i.

> +		}
> +
>  		global_addr = port->membase + UART_GLOBAL - IMX_REG_OFF;
>  		writel(UART_GLOBAL_RST, global_addr);
>  		usleep_range(GLOBAL_RST_MIN_US, GLOBAL_RST_MAX_US);
>  		writel(0, global_addr);
>  		usleep_range(GLOBAL_RST_MIN_US, GLOBAL_RST_MAX_US);
> +
> +		/* Recover the transmitter for earlycon */
> +		if (tx_enable) {
> +			lpuart32_write(port, bd, UARTBAUD);
> +			lpuart32_write(port, UARTCTRL_TE, UARTCTRL);
> +		}
>  	}
>  
>  	clk_disable_unprepare(sport->ipg_clk);
>