[PATCH net] net: lan966x: don't free XDP buffers in fdma tx clear with budget 0

[Daniel Machon]

lan966x_fdma_tx_clear_buf() runs from the NAPI poll and frees the
buffers of completed tx DCBs with napi_consume_skb(),
page_pool_recycle_direct() and xdp_return_frame_bulk().

netpoll calls the NAPI poll with a budget of 0 and hardirqs disabled.
With a budget of 0, a driver cannot call any XDP or page pool API.

Before this patch, the XDP_TX pages and ndo_xdp_xmit frames are freed
regardless of the budget. With netconsole on the port that owns the
FDMA NAPI, such a buffer is freed with hardirqs disabled, and
__page_pool_put_page() warns via lockdep_assert_no_hardirq().

Leave the XDP buffers of completed DCBs in place when the budget is 0,
and free them in the next NAPI poll. napi_consume_skb() already handles
a budget of 0, so skbs are still freed.

Found by an AI code review agent. Reproduced on a lan9668 board with a
modified kernel that runs the NAPI poll with a budget of 0 and hardirqs
disabled. With the fix the warning is gone.

Fixes: 19c6f534f636 ("net: lan966x: Add support for XDP_TX")
Signed-off-by: Daniel Machon <daniel.machon@xxxxxxxxxxxxx>
---
 drivers/net/ethernet/microchip/lan966x/lan966x_fdma.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/net/ethernet/microchip/lan966x/lan966x_fdma.c b/drivers/net/ethernet/microchip/lan966x/lan966x_fdma.c
index 21668bbcc9063..1fbe910c5ec38 100644
--- a/drivers/net/ethernet/microchip/lan966x/lan966x_fdma.c
+++ b/drivers/net/ethernet/microchip/lan966x/lan966x_fdma.c
@@ -353,6 +353,12 @@ static void lan966x_fdma_tx_clear_buf(struct lan966x *lan966x, int weight)
 		if (!fdma_db_is_done(db))
 			continue;
 
+		/* XDP and page pool APIs can't be used with a budget of 0.
+		 * Leave these buffers for the next NAPI poll.
+		 */
+		if (!weight && !dcb_buf->use_skb)
+			continue;
+
 		dcb_buf->dev->stats.tx_packets++;
 		dcb_buf->dev->stats.tx_bytes += dcb_buf->len;
 

---
base-commit: 829a09df899dcce860dcf5f70e09c1b2d7d425ab
change-id: 20261028-lan966x-fdma-tx-clear-budget-585aadb966b1

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