[PATCH] dmaengine: pl330: drop only the runtime PM reference a channel holds

["Krzysztof Sokołowski via B4 Relay" <devnull+krzysiek.sokolowski.gmail.com@kernel.org>]

From: Krzysztof Sokołowski <krzysiek.sokolowski@gmail.com>

pl330_tasklet() drops a runtime PM reference when the channel's work
list is empty, without checking that the channel still holds one.
pl330_terminate_all() drops that reference too (power_down =
pch->active) and clears pch->active. If a completion schedules the
tasklet and a client terminates the channel before the tasklet runs, the
tasklet finds an empty work list and drops the reference a second time:

  Runtime PM usage count underflow!

after which the controller can be runtime suspended while still in use.

Seen on an RK3128 board (dw_mmc using the PL330 for SD transfers) under
combined SD and USB load, where the dw_mmc error path terminates the
channel; the underflow was followed by "pl330_update:1678 Unexpected!"
and a failure to request the DMA channel again.

Take power_down from pch->active in the tasklet, as pl330_terminate_all()
does, so the reference taken in pl330_issue_pending() is dropped once.

Fixes: 5c9e6c2b2ba3 ("dmaengine: pl330: Fix runtime PM support for terminated transfers")
Cc: stable@vger.kernel.org
Signed-off-by: Krzysztof Sokołowski <krzysiek.sokolowski@gmail.com>
---
 drivers/dma/pl330.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/dma/pl330.c b/drivers/dma/pl330.c
index c2d0b47..b0eced8 100644
--- a/drivers/dma/pl330.c
+++ b/drivers/dma/pl330.c
@@ -2090,7 +2090,12 @@ static void pl330_tasklet(struct tasklet_struct *t)
 		spin_lock(&pch->thread->dmac->lock);
 		_stop(pch->thread);
 		spin_unlock(&pch->thread->dmac->lock);
-		power_down = true;
+		/*
+		 * Drop the runtime PM reference only if this channel still
+		 * holds it: pl330_terminate_all() may already have dropped it
+		 * while this tasklet was scheduled.
+		 */
+		power_down = pch->active;
 		pch->active = false;
 	} else {
 		/* Make sure the PL330 Channel thread is active */

---
base-commit: aff5e6a77fc69a7c490246465940d18422d7c631
change-id: 20261028-pl330-runtime-pm-b5a30050b2f5

Best regards,
--  
Krzysztof Sokołowski <krzysiek.sokolowski@gmail.com>