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Date:   Tue, 3 Aug 2021 17:07:50 +0300
From:   Andy Shevchenko <andriy.shevchenko@linux.intel.com>
To:     John Ogness <john.ogness@linutronix.de>
Cc:     Petr Mladek <pmladek@suse.com>,
        Sergey Senozhatsky <senozhatsky@chromium.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel@vger.kernel.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jiri Slaby <jirislaby@kernel.org>,
        Paul Cercueil <paul@crapouillou.net>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        Andrew Jeffery <andrew@aj.id.au>,
        Christophe JAILLET <christophe.jaillet@wanadoo.fr>,
        kuldip dwivedi <kuldip.dwivedi@puresoftware.com>,
        Wang Qing <wangqing@vivo.com>, Andrij Abyzov <aabyzov@slb.com>,
        Johan Hovold <johan@kernel.org>,
        Eddie Huang <eddie.huang@mediatek.com>,
        Claire Chang <tientzu@chromium.org>,
        Hsin-Yi Wang <hsinyi@chromium.org>,
        Zhang Qilong <zhangqilong3@huawei.com>,
        "Maciej W. Rozycki" <macro@orcam.me.uk>,
        Guenter Roeck <linux@roeck-us.net>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Serge Semin <Sergey.Semin@baikalelectronics.ru>,
        "Gustavo A. R. Silva" <gustavoars@kernel.org>,
        Al Cooper <alcooperx@gmail.com>, linux-serial@vger.kernel.org,
        linux-mips@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-mediatek@lists.infradead.org
Subject: Re: [PATCH printk v1 10/10] serial: 8250: implement write_atomic
Message-ID: <YQlNtr7TNAWtB8XF@smile.fi.intel.com>
References: <20210803131301.5588-1-john.ogness@linutronix.de>
 <20210803131301.5588-11-john.ogness@linutronix.de>
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In-Reply-To: <20210803131301.5588-11-john.ogness@linutronix.de>
Organization: Intel Finland Oy - BIC 0357606-4 - Westendinkatu 7, 02160 Espoo
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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