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

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Date:   Tue, 3 Aug 2021 17:07:50 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     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>,
        Jiri Slaby <jirislaby@...nel.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 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.

> +}

> +static inline unsigned char serial8250_clear_IER(struct uart_8250_port *up)
> +{
> +	struct uart_port *port = &up->port;
> +	unsigned int clearval = 0;
> +	unsigned long flags;
> +	unsigned int prior;
> +	bool is_console;
> +
> +	is_console = uart_console(port);
> +
> +	if (up->capabilities & UART_CAP_UUE)
> +		clearval = UART_IER_UUE;
> +
> +	if (is_console)
> +		console_atomic_cpu_lock(flags);
> +
> +	prior = serial_port_in(port, UART_IER);
> +	serial_port_out(port, UART_IER, clearval);
> +
> +	if (is_console)
> +		console_atomic_cpu_unlock(flags);

Ditto.

> +	return prior;
> +}

...

> +		is_console = uart_console(port);
> +
> +		if (is_console)
> +			console_atomic_cpu_lock(flags);
>  		up->ier = port->serial_in(port, UART_IER);
> +		if (is_console)
> +			console_atomic_cpu_unlock(flags);
> +

I'm wondering why you can't call above function here?

...

> +		is_console = uart_console(p);
> +		if (is_console)
> +			console_atomic_cpu_lock(flags);
>  		ier = p->serial_in(p, UART_IER);
> +		if (is_console)
> +			console_atomic_cpu_unlock(flags);

Ditto.

...

> +	is_console = uart_console(port);
> +
> +	if (is_console)
> +		console_atomic_cpu_lock(flags);
> +
> +	ier = serial_in(up, UART_IER);
> +	serial_out(up, UART_IER, ier & (~mask));
> +
> +	if (is_console)
> +		console_atomic_cpu_unlock(flags);

Ditto.

...

> +	if (uart_console(port))
> +		console_atomic_cpu_lock(flags);
> +
> +	ier = serial_in(up, UART_IER);
> +	serial_out(up, UART_IER, ier | mask);
> +
> +	if (uart_console(port))
> +		console_atomic_cpu_unlock(flags);

Ditto.

Looking into above note, that uart_console(port) can give different results
here, AFAIR.

-- 
With Best Regards,
Andy Shevchenko