[PATCH v2 1/2] hwrng: mtk - use pm_runtime_resume_and_get() in mtk_rng_read()

[Gu Bin]

mtk_rng_read() calls pm_runtime_get_sync() without checking the return
value and immediately starts polling and reading the RNG registers:
when resuming the device fails (negative return), the reads hit a
device that never resumed, and the failure is only surfaced indirectly
(if at all) through the -EIO fallback of the final return.

Use pm_runtime_resume_and_get() instead and bail out on the error: the
helper reports the resume failure and drops the usage counter reference
it took, which a failed pm_runtime_get_sync() leaves behind (the usage
counter is incremented before the resume attempt, so returning
directly after a failed get_sync() would leak the reference and the
device could never autosuspend again). The tail
pm_runtime_put_sync_autosuspend() must not be used for this bailout:
the device never resumed, so balancing the counter is all that is
needed. On success the helper returns 0, which is exactly what the
read counter expects at that point.

Fixes: 81d2b34508c6 ("hwrng: mtk - add runtime PM support")
Signed-off-by: Gu Bin <gubin@xxxxxxxxxxxxxxxxxxxx>
---
 drivers/char/hw_random/mtk-rng.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/char/hw_random/mtk-rng.c b/drivers/char/hw_random/mtk-rng.c
index 127b82d979e8..1e8e458dab25 100644
--- a/drivers/char/hw_random/mtk-rng.c
+++ b/drivers/char/hw_random/mtk-rng.c
@@ -95,7 +95,9 @@ static int mtk_rng_read(struct hwrng *rng, void *buf, size_t max, bool wait)
 	struct mtk_rng *priv = to_mtk_rng(rng);
 	int retval = 0;
 
-	pm_runtime_get_sync(priv->dev);
+	retval = pm_runtime_resume_and_get(priv->dev);
+	if (retval < 0)
+		return retval;
 
 	while (max >= sizeof(u32)) {
 		if (!mtk_rng_wait_ready(rng, wait))
-- 
2.55.0.windows.3