[PATCH 6.1 302/481] serial: uartlite: fix PM runtime usage count underflow on probe

Greg Kroah-Hartman
2026-03-23T16:20:31Z

6.1-stable review patch. If anyone has any objections, please let me know.

------------------

From: Maciej Andrzejewski ICEYE <maciej.andrzejewski@m-works.net>

commit d54801cd509515f674a5aac1d3ea1401d2a05863 upstream.

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 <stable@kernel.org>
Signed-off-by: Maciej Andrzejewski ICEYE <maciej.andrzejewski@m-works.net>
Link: https://patch.msgid.link/20260305123746.4152800-1-maciej.andrzejewski@m-works.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/tty/serial/uartlite.c | 1 +
1 file changed, 1 insertion(+)

--- a/drivers/tty/serial/uartlite.c
+++ b/drivers/tty/serial/uartlite.c
@@ -874,6 +874,7 @@ of_err:
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);