linux-kernel - Re: [PATCH tty v1 06/74] serial: 8250: Use port lock wrappers

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Date:   Fri, 15 Sep 2023 12:35:17 +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>,
        Tony Lindgren <tony@...mide.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Florian Fainelli <f.fainelli@...il.com>
Subject: Re: [PATCH tty v1 06/74] serial: 8250: 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_core.c |  12 ++--
>  drivers/tty/serial/8250/8250_port.c | 100 ++++++++++++++--------------
>  2 files changed, 56 insertions(+), 56 deletions(-)


> @@ -3403,9 +3403,9 @@ void serial8250_console_write(struct uart_8250_port *up, const char *s,
>  	touch_nmi_watchdog();
>  
>  	if (oops_in_progress)
> -		locked = spin_trylock_irqsave(&port->lock, flags);
> +		locked = uart_port_trylock_irqsave(port, &flags);
>  	else
> -		spin_lock_irqsave(&port->lock, flags);
> +		uart_port_lock_irqsave(port, &flags);

Not related to any problem (with this patch) but I'm a bit curious is this 
construct going to remain there after the follow-up work? And there's the 
similar one in some other drivers (with some variations related to 
local_irq_save()):

        if (port->sysrq) {
                locked = 0;
        } else if (oops_in_progress) {
                locked = spin_trylock(&port->lock);
        } else {
                spin_lock(&port->lock);
                locked = 1;
        }


-- 
 i.



>  	/*
>  	 *	First save the IER then disable the interrupts
> @@ -3475,7 +3475,7 @@ void serial8250_console_write(struct uart_8250_port *up, const char *s,
>  		serial8250_modem_status(up);
>  
>  	if (locked)
> -		spin_unlock_irqrestore(&port->lock, flags);
> +		uart_port_unlock_irqrestore(port, flags);
>  }
>  
>  static unsigned int probe_baud(struct uart_port *port)
>