linux-kernel - RE: [PATCH V2] tty: serial: fsl_lpuart: don't break the on-going transfer when global reset

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Date:   Wed, 19 Oct 2022 14:29:47 +0000
From:   Shenwei Wang <shenwei.wang@....com>
To:     Sherry Sun <sherry.sun@....com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "jirislaby@...nel.org" <jirislaby@...nel.org>,
        "lukas@...ner.de" <lukas@...ner.de>,
        "ilpo.jarvinen@...ux.intel.com" <ilpo.jarvinen@...ux.intel.com>
CC:     "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        dl-linux-imx <linux-imx@....com>
Subject: RE: [PATCH V2] tty: serial: fsl_lpuart: don't break the on-going
 transfer when global reset



> -----Original Message-----
> From: Sherry Sun <sherry.sun@....com>
> Sent: Wednesday, October 19, 2022 6:07 AM
> To: gregkh@...uxfoundation.org; jirislaby@...nel.org; lukas@...ner.de;
> ilpo.jarvinen@...ux.intel.com
> Cc: linux-serial@...r.kernel.org; linux-kernel@...r.kernel.org; dl-linux-imx
> <linux-imx@....com>
> Subject: [PATCH V2] tty: serial: fsl_lpuart: don't break the on-going transfer
> when global reset
> 
> 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>
> ---
> Changes in V2:
> 1. The while loop may never exit as the stat and sfifo are not re-read inside the
> loop, fix that.
> ---
>  drivers/tty/serial/fsl_lpuart.c | 22 +++++++++++++++++++---
>  1 file changed, 19 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/tty/serial/fsl_lpuart.c b/drivers/tty/serial/fsl_lpuart.c index
> 67fa113f77d4..9a0781395b1f 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;
>  	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 +418,29 @@ 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);
> +			while (!(lpuart32_read(port, UARTSTAT) &
> UARTSTAT_TC &&
> +				 lpuart32_read(port, UARTFIFO) &
> UARTFIFO_TXEMPT))
> +				cpu_relax();
> +		}
> +
>  		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);

According to the statement in the RM, you don't need to add delay here.
"There is no minimum delay required before clearing the software reset."

>  		writel(0, global_addr);
>  		usleep_range(GLOBAL_RST_MIN_US, GLOBAL_RST_MAX_US);

[...]

Regards,
Shenwei

> +
> +		/* 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);
> --
> 2.17.1