[PATCH v4 net 1/8] net: enetc: consistently track dropped frames in enetc_xdp_xmit()

[vladimir.oltean@nxp.com]

2 distinct classes of problems:
(a) the following events from enetc_xdp_xmit() will result in the frames
    being silently dropped without any counter tracking that action:
    - failures in enetc_xdp_frame_to_xdp_tx_swbd(),
    - frames split across too many buffers
    - frames enqueued while the interface is down
    Similar to how enetc_xdp_tx() increments xdp_tx_drops, let's reuse
    this ethtool counter for the above cases too.
(b) enetc_xdp_tx() failures increment xdp_tx_drops by 1, but that is not
    necessarily correct. Wei Fang points out that on .ndo_xdp_xmit()
    failure, bq_xmit_all() drops all remaining frames from the batch,
    not just the current one. So we should in any case increment the
    xdp_tx_drops counter by the remainder, not just by 1.

A limitation that remains is that the xdp_tx_drops ethtool statistics
counter (already) tracks XDP_TX and XDP_REDIRECT drops in the same
value. We should distinguish between these at some point, but that is an
ABI change and is out of scope for a bug fix.

Fixes: 9d2b68cc108d ("net: enetc: add support for XDP_REDIRECT")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
---
v3->v4:
- make sure the ENETC_TX_DOWN case also increments the xdp_tx_drops
  counter
- reword the commit message to enumerate the cases of silently dropped
  frames
v2->v3:
- patch is new; replaces:
https://lore.kernel.org/netdev/20260406204122.167237-2-vladimir.oltean@nxp.com/
---
 drivers/net/ethernet/freescale/enetc/enetc.c | 20 ++++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)

diff --git a/drivers/net/ethernet/freescale/enetc/enetc.c b/drivers/net/ethernet/freescale/enetc/enetc.c
index e91ba77a1532..103335f3d418 100644
--- a/drivers/net/ethernet/freescale/enetc/enetc.c
+++ b/drivers/net/ethernet/freescale/enetc/enetc.c
@@ -1811,15 +1811,18 @@ int enetc_xdp_xmit(struct net_device *ndev, int num_frames,
 	struct enetc_bdr *tx_ring;
 	int xdp_tx_bd_cnt, i, k;
 	int xdp_tx_frm_cnt = 0;
+	int err = 0;
+
+	tx_ring = priv->xdp_tx_ring[smp_processor_id()];
 
 	if (unlikely(test_bit(ENETC_TX_DOWN, &priv->flags) ||
-		     !netif_carrier_ok(ndev)))
-		return -ENETDOWN;
+		     !netif_carrier_ok(ndev))) {
+		err = -ENETDOWN;
+		goto out;
+	}
 
 	enetc_lock_mdio();
 
-	tx_ring = priv->xdp_tx_ring[smp_processor_id()];
-
 	prefetchw(ENETC_TXBD(*tx_ring, tx_ring->next_to_use));
 
 	for (k = 0; k < num_frames; k++) {
@@ -1840,7 +1843,6 @@ int enetc_xdp_xmit(struct net_device *ndev, int num_frames,
 			for (i = 0; i < xdp_tx_bd_cnt; i++)
 				enetc_unmap_tx_buff(tx_ring,
 						    &xdp_redirect_arr[i]);
-			tx_ring->stats.xdp_tx_drops++;
 			break;
 		}
 
@@ -1850,11 +1852,13 @@ int enetc_xdp_xmit(struct net_device *ndev, int num_frames,
 	if (unlikely((flags & XDP_XMIT_FLUSH) || k != xdp_tx_frm_cnt))
 		enetc_update_tx_ring_tail(tx_ring);
 
-	tx_ring->stats.xdp_tx += xdp_tx_frm_cnt;
-
 	enetc_unlock_mdio();
 
-	return xdp_tx_frm_cnt;
+out:
+	tx_ring->stats.xdp_tx += xdp_tx_frm_cnt;
+	tx_ring->stats.xdp_tx_drops += num_frames - xdp_tx_frm_cnt;
+
+	return err ?: xdp_tx_frm_cnt;
 }
 EXPORT_SYMBOL_GPL(enetc_xdp_xmit);
 
-- 
2.43.0