Serial console and interrupts latency.

[Sergey Organov <sorganov@gmail.com>]

Hello,

I was investigating random serial overruns on my embedded board and
figured it strongly correlates with serial output (to another serial
port) from kernel printk() calls, that forced me to dig into the kernel
sources, and now I'm very confused.

I'm reading drivers/tty/serial/8250/8250_port.c, and
serial8250_console_write() function in particular (being on tty-next
branch).

What I see is that it locks interrupts

3141:		spin_lock_irqsave(&port->lock, flags);

and then calls wait_for_xmitr() both indirectly here:

3159:	uart_console_write(port, s, count, serial8250_console_putchar);

and then directly as well:

3165:	wait_for_xmitr(up, BOTH_EMPTY);

before re-enabling interrupts at:

3179:		spin_unlock_irqrestore(&port->lock, flags);

Now, wait_for_xmitr(), even according to comments, could busy-wait for
up to 10+1000 milliseconds, and in this case this huge delay will happen
at interrupts disabled?

Does it mean any serial console output out of printk() could cause 10
milliseconds or even 1 second interrupts latency? Somehow I can't
believe it.

What do I miss?

Thanks,
-- Sergey