linux-kernel - [PATCH 2/4] NTB: ntb_transport: Ack DMA memcpy descriptors to avoid wait-list growth

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Message-ID: <20251027004331.562345-3-den@valinux.co.jp>
Date: Mon, 27 Oct 2025 09:43:29 +0900
From: Koichiro Den <den@...inux.co.jp>
To: ntb@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Cc: jdmason@...zu.us,
	dave.jiang@...el.com,
	allenbh@...il.com
Subject: [PATCH 2/4] NTB: ntb_transport: Ack DMA memcpy descriptors to avoid wait-list growth

ntb_transport prepares DMA memcpy transactions but never acks the
descriptors afterwards. On dmaengines that honor the ACK semantics
(e.g. rcar-dmac), completed descriptors are moved to the 'wait' list
and only recycled once they are ACK-ed. Since ntb_transport does not
chain, inspect residue, or retain transaction descriptors after
completion, we can mark them as ACK-ed at prep time.

Set DMA_CTRL_ACK when preparing RX/TX memcpy transfers so that the
engine can immediately recycle descriptors after completion. This
prevents unbounded growth of the wait list, which was observed on R-Car
S4 (rcar-dmac). Engines that ignore ACK or auto-recycle descriptors are
unaffected.

Signed-off-by: Koichiro Den <den@...inux.co.jp>
---
 drivers/ntb/ntb_transport.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/ntb/ntb_transport.c b/drivers/ntb/ntb_transport.c
index b9f9d2e0feb3..a447eca27d0f 100644
--- a/drivers/ntb/ntb_transport.c
+++ b/drivers/ntb/ntb_transport.c
@@ -1591,7 +1591,7 @@ static int ntb_async_rx_submit(struct ntb_queue_entry *entry, void *offset)

 	txd = device->device_prep_dma_memcpy(chan, unmap->addr[1],
 					     unmap->addr[0], len,
-					     DMA_PREP_INTERRUPT);
+					     DMA_PREP_INTERRUPT | DMA_CTRL_ACK);
 	if (!txd)
 		goto err_get_unmap;

@@ -1864,7 +1864,7 @@ static int ntb_async_tx_submit(struct ntb_transport_qp *qp,
 	unmap->to_cnt = 1;

 	txd = device->device_prep_dma_memcpy(chan, dest, unmap->addr[0], len,
-					     DMA_PREP_INTERRUPT);
+					     DMA_PREP_INTERRUPT | DMA_CTRL_ACK);
 	if (!txd)
 		goto err_get_unmap;

-- 
2.48.1