[PATCH] serial: uartlite: fix PM runtime usage count underflow on probe

[Maciej Andrzejewski ICEYE <maciej.andrzejewski@m-works.net>]

ulite_probe() calls pm_runtime_put_autosuspend() at the end of probe
without holding a corresponding PM runtime reference for non-console
ports.

During ulite_assign(), uart_add_one_port() triggers uart_configure_port()
which calls ulite_pm() via uart_change_pm(). For non-console ports, the
UART core performs a balanced get/put cycle:

  uart_change_pm(ON)  -> ulite_pm() -> pm_runtime_get_sync()        +1
  uart_change_pm(OFF) -> ulite_pm() -> pm_runtime_put_autosuspend() -1

This leaves no spare reference for the pm_runtime_put_autosuspend() at
the end of probe. The PM runtime core prevents the count from actually
going below zero, and instead triggers a
"Runtime PM usage count underflow!" warning.

For console ports the bug is masked: the UART core skips the
uart_change_pm(OFF) call, so the UART core's unbalanced get happens to
pair with probe's trailing put.

Add pm_runtime_get_noresume() before pm_runtime_enable() to take an
explicit probe-owned reference that the trailing
pm_runtime_put_autosuspend() can release. This ensures a correct usage
count regardless of whether the port is a console.

Fixes: 5bbe10a6942d ("tty: serial: uartlite: Add runtime pm support")
Cc: stable@vger.kernel.org
Signed-off-by: Maciej Andrzejewski ICEYE <maciej.andrzejewski@m-works.net>
---
 drivers/tty/serial/uartlite.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/tty/serial/uartlite.c b/drivers/tty/serial/uartlite.c
index 39c1fd1ff9ce..6240c3d4dfd7 100644
--- a/drivers/tty/serial/uartlite.c
+++ b/drivers/tty/serial/uartlite.c
@@ -878,6 +878,7 @@ static int ulite_probe(struct platform_device *pdev)
 	pm_runtime_use_autosuspend(&pdev->dev);
 	pm_runtime_set_autosuspend_delay(&pdev->dev, UART_AUTOSUSPEND_TIMEOUT);
 	pm_runtime_set_active(&pdev->dev);
+	pm_runtime_get_noresume(&pdev->dev);
 	pm_runtime_enable(&pdev->dev);
 
 	ret = ulite_assign(&pdev->dev, id, res->start, irq, pdata);
-- 
2.43.0