[PATCH net] net: macb: exclude software FCS from TX byte statistics
From: Nicolai Buchwitz <hidden>
Date: 2026-08-31 11:31:53
Also in:
lkml
Subsystem:
atmel macb ethernet driver, networking drivers, the rest · Maintainers:
Théo Lebrun, Andrew Lunn, "David S. Miller", Eric Dumazet, Jakub Kicinski, Paolo Abeni, Linus Torvalds
Frames for which macb_pad_and_fcs() supplies the FCS have four FCS
bytes appended, and TX completion then accounts the grown skb->len.
tx_bytes is defined to exclude the FCS, so these frames are reported
four bytes too large.
Track only the number of FCS bytes appended in software, 0 or
ETH_FCS_LEN, and subtract that from skb->len at completion. skb->len
already reflects the padded length by then, so there is nothing else
to store. macb_pad_and_fcs() already returns 0 on every non-error
path. Return the FCS length from there instead, rather than
recomputing the same check in the caller. BQL stays on the padded
skb->len that netdev_tx_sent_queue() saw.
Fixes: 653e92a9175e ("net: macb: add support for padding and fcs computation")
Signed-off-by: Nicolai Buchwitz <redacted>
---
Originally patch 1/2 of "net: macb: fix zero UDPv4 checksum on
transmit" [1], split out into its own patch since it's a pre-existing,
partly related issue, as suggested by Paolo and David.
[1] https://lore.kernel.org/all/20260824134703.766708-1-nb@tipi-net.de/ (local)
drivers/net/ethernet/cadence/macb.h | 3 +++
drivers/net/ethernet/cadence/macb_main.c | 21 +++++++++++++--------
2 files changed, 16 insertions(+), 8 deletions(-)
diff --git a/drivers/net/ethernet/cadence/macb.h b/drivers/net/ethernet/cadence/macb.h
index 1e1f52285a39..d6931c41f39d 100644
--- a/drivers/net/ethernet/cadence/macb.h
+++ b/drivers/net/ethernet/cadence/macb.h@@ -968,6 +968,8 @@ struct macb_dma_desc_ptp { * of the frame * @mapping: DMA address of the skb's fragment buffer * @size: size of the DMA mapped buffer + * @fcs_len: FCS bytes appended in software, 0 or ETH_FCS_LEN, only + * set for the last buffer of the frame * @mapped_as_page: true when buffer was mapped with skb_frag_dma_map(), * false when buffer was mapped with dma_map_single() */
@@ -975,6 +977,7 @@ struct macb_tx_skb { struct sk_buff *skb; dma_addr_t mapping; size_t size; + u8 fcs_len; bool mapped_as_page; };
diff --git a/drivers/net/ethernet/cadence/macb_main.c b/drivers/net/ethernet/cadence/macb_main.c
index 76ee4f506033..b1939da4c95a 100644
--- a/drivers/net/ethernet/cadence/macb_main.c
+++ b/drivers/net/ethernet/cadence/macb_main.c@@ -1322,8 +1322,8 @@ static void macb_tx_error_task(struct work_struct *work) bp->netdev->stats.tx_packets++; queue->stats.tx_packets++; packets++; - bp->netdev->stats.tx_bytes += skb->len; - queue->stats.tx_bytes += skb->len; + bp->netdev->stats.tx_bytes += skb->len - tx_skb->fcs_len; + queue->stats.tx_bytes += skb->len - tx_skb->fcs_len; bytes += skb->len; } } else {
@@ -1450,8 +1450,8 @@ static int macb_tx_complete(struct macb_queue *queue, int budget) skb->data); bp->netdev->stats.tx_packets++; queue->stats.tx_packets++; - bp->netdev->stats.tx_bytes += skb->len; - queue->stats.tx_bytes += skb->len; + bp->netdev->stats.tx_bytes += skb->len - tx_skb->fcs_len; + queue->stats.tx_bytes += skb->len - tx_skb->fcs_len; packets++; bytes += skb->len; }
@@ -2199,7 +2199,8 @@ static void macb_poll_controller(struct net_device *netdev) static unsigned int macb_tx_map(struct macb *bp, struct macb_queue *queue, struct sk_buff *skb, - unsigned int hdrlen) + unsigned int hdrlen, + u8 fcs_len) { unsigned int f, nr_frags = skb_shinfo(skb)->nr_frags; unsigned int len, i, tx_head = queue->tx_head;
@@ -2284,6 +2285,7 @@ static unsigned int macb_tx_map(struct macb *bp, /* This is the last buffer of the frame: save socket buffer */ tx_skb->skb = skb; + tx_skb->fcs_len = fcs_len; /* Update TX ring: update buffer descriptors in reverse order * to avoid race condition
@@ -2417,6 +2419,7 @@ static inline int macb_clear_csum(struct sk_buff *skb) return 0; } +/* Returns a negative errno, or the FCS bytes appended (0 or ETH_FCS_LEN). */ static int macb_pad_and_fcs(struct sk_buff **skb, struct net_device *netdev) { bool cloned = skb_cloned(*skb) || skb_header_cloned(*skb) ||
@@ -2465,7 +2468,7 @@ static int macb_pad_and_fcs(struct sk_buff **skb, struct net_device *netdev) skb_put_u8(*skb, (fcs >> 16) & 0xff); skb_put_u8(*skb, (fcs >> 24) & 0xff); - return 0; + return ETH_FCS_LEN; } static netdev_tx_t macb_start_xmit(struct sk_buff *skb,
@@ -2478,6 +2481,7 @@ static netdev_tx_t macb_start_xmit(struct sk_buff *skb, netdev_tx_t ret = NETDEV_TX_OK; unsigned int hdrlen; unsigned long flags; + int fcs_len; bool is_lso; if (macb_clear_csum(skb)) {
@@ -2485,7 +2489,8 @@ static netdev_tx_t macb_start_xmit(struct sk_buff *skb, return ret; } - if (macb_pad_and_fcs(&skb, netdev)) { + fcs_len = macb_pad_and_fcs(&skb, netdev); + if (fcs_len < 0) { dev_kfree_skb_any(skb); return ret; }
@@ -2548,7 +2553,7 @@ static netdev_tx_t macb_start_xmit(struct sk_buff *skb, } /* Map socket buffer for DMA transfer */ - if (macb_tx_map(bp, queue, skb, hdrlen)) { + if (macb_tx_map(bp, queue, skb, hdrlen, fcs_len)) { dev_kfree_skb_any(skb); goto unlock; }
--
2.53.0