[PATCH net] net: sparx5: don't reuse the last DCB in fdma xmit

[Daniel Machon]

sparx5_fdma_xmit() advances dcb_index to the next tx DCB and, if the
FDMA is done with it, adds it to the end of the chain, which makes it
the new fdma->last_dcb.

Before this patch, sparx5_fdma_xmit() can pick last_dcb itself. A
failed transmit advances dcb_index without adding a DCB, so dcb_index
no longer follows last_dcb, and after a burst of failures it can land
on it. Re-adding last_dcb loses the frame, because the FDMA has already
passed the tail of the chain.

Also reject the DCB when it is the last one in the chain, using the
same !fdma_is_last() guard that lan966x and lan969x use to pick their
next tx DCB, so the tail is never re-added.

Found by an AI code review agent and confirmed by code inspection; not
reproduced. Tested on sparx5 that traffic still passes, with ping floods
in both directions.

Fixes: f4aa7e361ae2 ("net: sparx5: use FDMA library for adding DCB's in the tx path")
Signed-off-by: Daniel Machon <daniel.machon@xxxxxxxxxxxxx>
---
 drivers/net/ethernet/microchip/sparx5/sparx5_fdma.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/microchip/sparx5/sparx5_fdma.c b/drivers/net/ethernet/microchip/sparx5/sparx5_fdma.c
index dbe86f937b21..221b2f1932f6 100644
--- a/drivers/net/ethernet/microchip/sparx5/sparx5_fdma.c
+++ b/drivers/net/ethernet/microchip/sparx5/sparx5_fdma.c
@@ -218,7 +218,8 @@ int sparx5_fdma_xmit(struct sparx5 *sparx5, u32 *ifh, struct sk_buff *skb,
 	void *virt_addr;
 
 	fdma_dcb_advance(fdma);
-	if (!fdma_db_is_done(fdma_db_get(fdma, fdma->dcb_index, 0)))
+	if (!fdma_db_is_done(fdma_db_get(fdma, fdma->dcb_index, 0)) ||
+	    fdma_is_last(fdma, fdma_dcb_next_get(fdma)))
 		return -EINVAL;
 
 	/* Get the virtual address of the dataptr for the next DB */

---
base-commit: 07bd391eb9572e74f073c49acef327611234f9bb
change-id: 20261019-sparx5-fdma-last-dcb-e2e8491b7d86

Best regards,
-- 
Daniel Machon <daniel.machon@xxxxxxxxxxxxx>