Re: [PATCH printk v1 10/10] serial: 8250: implement write_atomic

Andy Shevchenko
2021-08-03T14:08:09Z

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