linux-kernel - Re: [PATCH] serial: imx: fix cached UCR2 read on software reset

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Date:   Thu, 19 Apr 2018 23:37:23 +0200
From:   Stefan Agner <stefan@...er.ch>
To:     u.kleine-koenig@...gutronix.de
Cc:     gregkh@...uxfoundation.org, jslaby@...e.com,
        linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] serial: imx: fix cached UCR2 read on software reset

Hi Uwe,

On 16.04.2018 17:35, Stefan Agner wrote:
> To reset the UART the SRST needs be cleared (low active). According
> to the documentation the bit will remain active for 4 module clocks
> until it is cleared (set to 1).
> 
> Hence the real register need to be read in case the cached register
> indcates that the SRST bit is zero.
> 
> This bug lead to wrong baudrate because the baud rate register got
> restored before reset completed in imx_flush_buffer.

Given that you reviewed my other patch rather quickly, you might have
overlooked this one?

Since it is a regression, this should go into v4.17 still...

--
Stefan

> 
> Fixes: 3a0ab62f43de ("serial: imx: implement shadow registers for UCRx
> and UFCR")
> Signed-off-by: Stefan Agner <stefan@...er.ch>
> ---
>  drivers/tty/serial/imx.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/serial/imx.c b/drivers/tty/serial/imx.c
> index 91f3a1a5cb7f..4ff6bd6eb9ab 100644
> --- a/drivers/tty/serial/imx.c
> +++ b/drivers/tty/serial/imx.c
> @@ -316,7 +316,7 @@ static u32 imx_uart_readl(struct imx_port *sport,
> u32 offset)
>  		 * differ from the value that was last written. As it only
>  		 * clears after being set, reread conditionally.
>  		 */
> -		if (sport->ucr2 & UCR2_SRST)
> +		if (!(sport->ucr2 & UCR2_SRST))
>  			sport->ucr2 = readl(sport->port.membase + offset);
>  		return sport->ucr2;
>  		break;