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

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Date:   Fri, 15 Sep 2023 13:27:05 +0206
From:   John Ogness <john.ogness@...utronix.de>
To:     Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
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 2023-09-15, Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com> wrote:
>> @@ -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?

Yes. The uart port lock already provides excellent coverage of unsafe
regions in uart drivers. We want to take advantage of that.

> 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;
>         }

With the follow-up work we are introducing a new type of console
(CON_NBCON) that supports atomic and threaded printing. Current console
drivers must be converted if they want these features. When converting a
driver to NBCON, such variations as above will need to be addressed. The
follow-up work provides new functions and semantics to allow drivers to
implement more reliable code than just: "trylock and keep going no
matter what".

For console drivers that are _not_ converted to NBCON, the uart port
lock wrappers will not provide any changed functionality.

John Ogness