# \[PATCH 15/16\] NTB: ntb\_transport: Report the direct-DMA payload limit

- From: Koichiro Den \<hidden\>
- Date: 2026-08-10 16:52:05 
- List: netdev
- Page: https://ratatoskr.run/netdev/2026/08/17392811
- Thread: https://ratatoskr.run/netdev/2026/08/17392799/t.md

```text
ntb_transport_max_size() currently reports the copy-path limit, which
reserves space for struct ntb_payload_header. Direct DMA bypasses that
layout, so the same limit is unnecessarily restrictive.

For active direct-DMA TX, derive the limit from the DMA device's segment
and mapping limits. Keep it within INT_MAX because transport callbacks
report lengths as int. Continue reporting the copy-path limit until direct
TX is active.

This lets ntb_netdev use ETH_MAX_MTU over direct DMA. If the queue later
falls back to the copy path, larger frames are rejected.

Signed-off-by: Koichiro Den <redacted>
---
 drivers/ntb/ntb_transport.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/ntb/ntb_transport.c b/drivers/ntb/ntb_transport.c
index 9f5d6a407be7..2f69328d5201 100644
--- a/drivers/ntb/ntb_transport.c
+++ b/drivers/ntb/ntb_transport.c
@@ -3787,10 +3787,19 @@ unsigned int ntb_transport_max_size(struct ntb_transport_qp *qp)
 	unsigned int max_size;
 	unsigned int copy_align;
 	struct dma_chan *rx_chan, *tx_chan;
+	struct device *dma_dev;
 
 	if (!qp)
 		return 0;
 
+	if (ntb_transport_link_query(qp) && ntb_direct_tx_enabled(qp)) {
+		dma_dev = dmaengine_get_dma_device(qp->direct_dma_chan);
+		max_size = min_t(size_t, dma_get_max_seg_size(dma_dev),
+				 dma_max_mapping_size(dma_dev));
+
+		return min_t(unsigned int, max_size, INT_MAX);
+	}
+
 	rx_chan = qp->rx_dma_chan;
 	tx_chan = qp->tx_dma_chan;
 
-- 
2.51.0

```