linux-kernel - Re: [PATCH printk v1 10/10] serial: 8250: implement write

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Date:   Thu, 5 Aug 2021 09:47:12 +0200
From:   Jiri Slaby <jirislaby@...nel.org>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        John Ogness <john.ogness@...utronix.de>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Paul Cercueil <paul@...pouillou.net>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Andrew Jeffery <andrew@...id.au>,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        kuldip dwivedi <kuldip.dwivedi@...esoftware.com>,
        Wang Qing <wangqing@...o.com>, Andrij Abyzov <aabyzov@....com>,
        Johan Hovold <johan@...nel.org>,
        Eddie Huang <eddie.huang@...iatek.com>,
        Claire Chang <tientzu@...omium.org>,
        Hsin-Yi Wang <hsinyi@...omium.org>,
        Zhang Qilong <zhangqilong3@...wei.com>,
        "Maciej W. Rozycki" <macro@...am.me.uk>,
        Guenter Roeck <linux@...ck-us.net>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Serge Semin <Sergey.Semin@...kalelectronics.ru>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Al Cooper <alcooperx@...il.com>, linux-serial@...r.kernel.org,
        linux-mips@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org
Subject: Re: [PATCH printk v1 10/10] serial: 8250: implement write_atomic

On 03. 08. 21, 16:07, Andy Shevchenko wrote:
> On Tue, Aug 03, 2021 at 03:19:01PM +0206, John Ogness wrote:
>> Implement an NMI-safe write_atomic() console function in order to
>> support synchronous console printing.
>>
>> Since interrupts need to be disabled during transmit, all usage of
>> the IER register is wrapped with access functions that use the
>> printk cpulock to synchronize register access while tracking the
>> state of the interrupts. This is necessary because write_atomic()
>> can be called from an NMI context that has preempted write_atomic().
> 
> ...
> 
>> +static inline void serial8250_set_IER(struct uart_8250_port *up,
>> +				      unsigned char ier)
>> +{
>> +	struct uart_port *port = &up->port;
>> +	unsigned long flags;
>> +	bool is_console;
> 
>> +	is_console = uart_console(port);
>> +
>> +	if (is_console)
>> +		console_atomic_cpu_lock(flags);
>> +
>> +	serial_out(up, UART_IER, ier);
>> +
>> +	if (is_console)
>> +		console_atomic_cpu_unlock(flags);
> 
> I would rewrite it as
> 
> 	if (uart_console()) {
> 		console_atomic_cpu_lock(flags);
> 		serial_out(up, UART_IER, ier);
> 		console_atomic_cpu_unlock(flags);
> 	} else {
> 		serial_out(up, UART_IER, ier);
> 	}
> 
> No additional variable, easier to get the algorithm on the first glance, less
> error prone.

Yes, the original is terrible.

Another option:

bool locked = console_atomic_cpu_lock(flags, uart_console());
serial_out(up, UART_IER, ier);
console_atomic_cpu_unlock(flags, locked);


Which makes console_atomic_cpu_lock to lock only if second parameter is 
true and return its value too.

BTW I actually don't know what console_atomic_cpu_lock does to think 
about it more as I was not CCed, and neither lore sees the other patches:
https://lore.kernel.org/linux-mips/20210803131301.5588-1-john.ogness@linutronix.de/

thanks,
-- 
js
suse labs