[PATCH v2] hwrng: stm32: fix usage_count leak when autosuspend_delay is negative

[Guangshuo Li]

stm32_rng_probe() calls pm_runtime_use_autosuspend(), but runtime PM is
enabled with pm_runtime_enable() and the matching
pm_runtime_dont_use_autosuspend() is not called on driver teardown.

If the autosuspend delay is set to a negative value while autosuspend
is enabled, the runtime PM core increments usage_count to prevent
runtime suspend. Without calling pm_runtime_dont_use_autosuspend()
during teardown, this reference is not dropped and usage_count remains
unbalanced.

Use devm_pm_runtime_enable() so that pm_runtime_dont_use_autosuspend()
and pm_runtime_disable() are automatically called on probe failure and
driver teardown. With runtime PM cleanup handled by devres,
stm32_rng_remove() is no longer needed.

This issue was found by manual code inspection.

Fixes: c6a97c42e399 ("hwrng: stm32 - add support for STM32 HW RNG")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Guangshuo Li <lgs201920130244@xxxxxxxxx>
---
v2:
  - Replace pm_runtime_enable() with devm_pm_runtime_enable() to handle
    runtime PM cleanup through devres.
  - Remove stm32_rng_remove() and the manual cleanup on the probe failure
    path.

 drivers/char/hw_random/stm32-rng.c | 10 +++-------
 1 file changed, 3 insertions(+), 7 deletions(-)

diff --git a/drivers/char/hw_random/stm32-rng.c b/drivers/char/hw_random/stm32-rng.c
index 9a8c00586ab0..f5bfe54c01dc 100644
--- a/drivers/char/hw_random/stm32-rng.c
+++ b/drivers/char/hw_random/stm32-rng.c
@@ -368,11 +368,6 @@ static int stm32_rng_init(struct hwrng *rng)
 	return 0;
 }
 
-static void stm32_rng_remove(struct platform_device *ofdev)
-{
-	pm_runtime_disable(&ofdev->dev);
-}
-
 static int __maybe_unused stm32_rng_runtime_suspend(struct device *dev)
 {
 	struct stm32_rng_private *priv = dev_get_drvdata(dev);
@@ -590,7 +585,9 @@ static int stm32_rng_probe(struct platform_device *ofdev)
 
 	pm_runtime_set_autosuspend_delay(dev, 100);
 	pm_runtime_use_autosuspend(dev);
-	pm_runtime_enable(dev);
+	ret = devm_pm_runtime_enable(dev);
+	if (ret)
+		return ret;
 
 	return devm_hwrng_register(dev, &priv->rng);
 }
@@ -602,7 +599,6 @@ static struct platform_driver stm32_rng_driver = {
 		.of_match_table = stm32_rng_match,
 	},
 	.probe = stm32_rng_probe,
-	.remove = stm32_rng_remove,
 };
 
 module_platform_driver(stm32_rng_driver);
-- 
2.43.0