When console is enabled, univ8250_console_setup() calls
serial8250_console_setup() before .dev is set to uart_port. Therefore,
it will not call pm_runtime_get_sync(). Later, when the actual driver
is going to take over univ8250_console_exit() is called. As .dev is
already set, serial8250_console_exit() makes pm_runtime_put_sync() call
with usage count being zero triggering PM usage count warning
(extra debug for univ8250_console_setup(), univ8250_console_exit(), and
serial8250_register_ports()):

[    0.068987] univ8250_console_setup ttyS0 nodev
[    0.499670] printk: console [ttyS0] enabled
[    0.717955] printk: console [ttyS0] printing thread started
[    1.960163] serial8250_register_ports assigned dev for ttyS0
[    1.976830] printk: console [ttyS0] disabled
[    1.976888] printk: console [ttyS0] printing thread stopped
[    1.977073] univ8250_console_exit ttyS0 usage:0
[    1.977075] serial8250 serial8250: Runtime PM usage count underflow!
[    1.977429] dw-apb-uart.6: ttyS0 at MMIO 0x4010006000 (irq = 33, base_baud = 115200) is a 16550A
[    1.977812] univ8250_console_setup ttyS0 usage:2
[    1.978167] printk: console [ttyS0] printing thread started
[    1.978203] printk: console [ttyS0] enabled

To fix the issue, call pm_runtime_get_sync() in
serial8250_register_ports() as soon as .dev is set for an uart_port
if it has console enabled.

This problem became apparent only recently because 82586a72 ("PM:
runtime: Avoid device usage count underflows") added the warning
printout. I confirmed this problem also occurs with v5.18 (w/o the
warning printout, obviously).

Fixes: bedb404e ("serial: 8250_port: Don't use power management for kernel console")
Cc: stable <stable@kernel.org>
Tested-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/b4f428e9-491f-daf2-2232-819928dc276e@linux.intel.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>