linux-kernel - Re: [PATCH tty v1 08/74] serial: 8250

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Date:   Fri, 15 Sep 2023 12:16:45 +0300 (EEST)
From:   Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To:     John Ogness <john.ogness@...utronix.de>
cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        linux-serial <linux-serial@...r.kernel.org>,
        Petr Mladek <pmladek@...e.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Subject: Re: [PATCH tty v1 08/74] serial: 8250_dw: Use port lock wrappers

On Thu, 14 Sep 2023, John Ogness wrote:

> From: Thomas Gleixner <tglx@...utronix.de>
> 
> When a serial port is used for kernel console output, then all
> modifications to the UART registers which are done from other contexts,
> e.g. getty, termios, are interference points for the kernel console.
> 
> So far this has been ignored and the printk output is based on the
> principle of hope. The rework of the console infrastructure which aims to
> support threaded and atomic consoles, requires to mark sections which
> modify the UART registers as unsafe. This allows the atomic write function
> to make informed decisions and eventually to restore operational state. It
> also allows to prevent the regular UART code from modifying UART registers
> while printk output is in progress.
> 
> All modifications of UART registers are guarded by the UART port lock,
> which provides an obvious synchronization point with the console
> infrastructure.
> 
> To avoid adding this functionality to all UART drivers, wrap the
> spin_[un]lock*() invocations for uart_port::lock into helper functions
> which just contain the spin_[un]lock*() invocations for now. In a
> subsequent step these helpers will gain the console synchronization
> mechanisms.
> 
> Converted with coccinelle. No functional change.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> ---
>  drivers/tty/serial/8250/8250_dw.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/tty/serial/8250/8250_dw.c b/drivers/tty/serial/8250/8250_dw.c
> index f4cafca1a7da..95d45dce0880 100644
> --- a/drivers/tty/serial/8250/8250_dw.c
> +++ b/drivers/tty/serial/8250/8250_dw.c
> @@ -263,20 +263,20 @@ static int dw8250_handle_irq(struct uart_port *p)
>  	 * so we limit the workaround only to non-DMA mode.
>  	 */
>  	if (!up->dma && rx_timeout) {
> -		spin_lock_irqsave(&p->lock, flags);
> +		uart_port_lock_irqsave(p, &flags);
>  		status = serial_lsr_in(up);
>  
>  		if (!(status & (UART_LSR_DR | UART_LSR_BI)))
>  			(void) p->serial_in(p, UART_RX);
>  
> -		spin_unlock_irqrestore(&p->lock, flags);
> +		uart_port_unlock_irqrestore(p, flags);
>  	}
>  
>  	/* Manually stop the Rx DMA transfer when acting as flow controller */
>  	if (quirks & DW_UART_QUIRK_IS_DMA_FC && up->dma && up->dma->rx_running && rx_timeout) {
> -		spin_lock_irqsave(&p->lock, flags);
> +		uart_port_lock_irqsave(p, &flags);
>  		status = serial_lsr_in(up);
> -		spin_unlock_irqrestore(&p->lock, flags);
> +		uart_port_unlock_irqrestore(p, flags);
>  
>  		if (status & (UART_LSR_DR | UART_LSR_BI)) {
>  			dw8250_writel_ext(p, RZN1_UART_RDMACR, 0);
> 

Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>

-- 
 i.