Subject: Re: [PATCH net 2/2] net: xilinx: axienet: Fix BQL accounting for multi-BD TX packets

From: Sean Anderson <[email-masked]>
Date: Tue, 24 Mar 2026 12:09:04 -0400

On 3/24/26 10:53, Suraj Gupta wrote:
> When a TX packet spans multiple buffer descriptors (scatter-gather),
> the per-BD byte count is accumulated into a local variable that resets
> on each NAPI poll. If the BDs for a single packet complete across
> different polls, the earlier bytes are lost and never credited to BQL.
> This causes BQL to think bytes are permanently in-flight, eventually
> stalling the TX queue.
> 
> Fix this by replacing the local accumulator with a persistent counter
> (tx_compl_bytes) that survives across polls and is reset only after
> updating BQL and stats.

Do we need this? Can't we just do something like

diff --git a/drivers/net/ethernet/xilinx/xilinx_axienet_main.c b/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
index 415e9bc252527..1ea8a6592bce1 100644
--- a/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
+++ b/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
@@ -768,6 +768,7 @@ static int axienet_free_tx_chain(struct axienet_local *lp, u32 *sizep, int budge
                if (cur_p->skb) {
                        struct axienet_cb *cb = (void *)cur_p->skb->cb;
 
+                       *sizep += skb->len;
                        dma_unmap_sgtable(lp->dev, &cb->sgt, DMA_TO_DEVICE, 0);
                        sg_free_table_chained(&cb->sgt, XAE_INLINE_SG_CNT);
                        napi_consume_skb(cur_p->skb, budget);
@@ -783,8 +784,6 @@ static int axienet_free_tx_chain(struct axienet_local *lp, u32 *sizep, int budge
                wmb();
                cur_p->cntrl = 0;
                cur_p->status = 0;
-
-               *sizep += status & XAXIDMA_BD_STS_ACTUAL_LEN_MASK;
        }
 
        smp_store_release(&lp->tx_bd_ci, (ci + i) & (lp->tx_bd_num - 1));

> Fixes: c900e49d58eb ("net: xilinx: axienet: Implement BQL")
> Signed-off-by: Suraj Gupta <[email-masked]>
> ---
>  drivers/net/ethernet/xilinx/xilinx_axienet.h  |  3 +++
>  .../net/ethernet/xilinx/xilinx_axienet_main.c | 20 +++++++++----------
>  2 files changed, 13 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/net/ethernet/xilinx/xilinx_axienet.h b/drivers/net/ethernet/xilinx/xilinx_axienet.h
> index 602389843342..a4444c939451 100644
> --- a/drivers/net/ethernet/xilinx/xilinx_axienet.h
> +++ b/drivers/net/ethernet/xilinx/xilinx_axienet.h
> @@ -509,6 +509,8 @@ struct skbuf_dma_descriptor {
>   *		complete. Only updated at runtime by TX NAPI poll.
>   * @tx_bd_tail:	Stores the index of the next Tx buffer descriptor in the ring
>   *              to be populated.
> + * @tx_compl_bytes: Accumulates TX completion length until a full packet is
> + *              reported to the stack.
>   * @tx_packets: TX packet count for statistics
>   * @tx_bytes:	TX byte count for statistics
>   * @tx_stat_sync: Synchronization object for TX stats
> @@ -592,6 +594,7 @@ struct axienet_local {
>  	u32 tx_bd_num;
>  	u32 tx_bd_ci;
>  	u32 tx_bd_tail;
> +	u32 tx_compl_bytes;
>  	u64_stats_t tx_packets;
>  	u64_stats_t tx_bytes;
>  	struct u64_stats_sync tx_stat_sync;
> diff --git a/drivers/net/ethernet/xilinx/xilinx_axienet_main.c b/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
> index b06e4c37ff61..95bf61986cb7 100644
> --- a/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
> +++ b/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
> @@ -692,6 +692,8 @@ static void axienet_dma_stop(struct axienet_local *lp)
>  	axienet_lock_mii(lp);
>  	__axienet_device_reset(lp);
>  	axienet_unlock_mii(lp);
> +
> +	lp->tx_compl_bytes = 0;
>  }
>  
>  /**
> @@ -770,8 +772,6 @@ static int axienet_device_reset(struct net_device *ndev)
>   * @first_bd:	Index of first descriptor to clean up
>   * @nr_bds:	Max number of descriptors to clean up
>   * @force:	Whether to clean descriptors even if not complete
> - * @sizep:	Pointer to a u32 filled with the total sum of all bytes
> - *		in all cleaned-up descriptors. Ignored if NULL.
>   * @budget:	NAPI budget (use 0 when not called from NAPI poll)
>   *
>   * Would either be called after a successful transmit operation, or after
> @@ -780,7 +780,7 @@ static int axienet_device_reset(struct net_device *ndev)
>   * Return: The number of packets handled.
>   */
>  static int axienet_free_tx_chain(struct axienet_local *lp, u32 first_bd,
> -				 int nr_bds, bool force, u32 *sizep, int budget)
> +				 int nr_bds, bool force, int budget)
>  {
>  	struct axidma_bd *cur_p;
>  	unsigned int status;
> @@ -819,8 +819,8 @@ static int axienet_free_tx_chain(struct axienet_local *lp, u32 first_bd,
>  		cur_p->cntrl = 0;
>  		cur_p->status = 0;
>  
> -		if (sizep)
> -			*sizep += status & XAXIDMA_BD_STS_ACTUAL_LEN_MASK;
> +		if (!force)
> +			lp->tx_compl_bytes += status & XAXIDMA_BD_STS_ACTUAL_LEN_MASK;
>  	}
>  
>  	if (!force) {
> @@ -999,18 +999,18 @@ static int axienet_tx_poll(struct napi_struct *napi, int budget)
>  {
>  	struct axienet_local *lp = container_of(napi, struct axienet_local, napi_tx);
>  	struct net_device *ndev = lp->ndev;
> -	u32 size = 0;
>  	int packets;
>  
>  	packets = axienet_free_tx_chain(lp, lp->tx_bd_ci, lp->tx_bd_num, false,
> -					&size, budget);
> +					budget);
>  
>  	if (packets) {
> -		netdev_completed_queue(ndev, packets, size);
> +		netdev_completed_queue(ndev, packets, lp->tx_compl_bytes);
>  		u64_stats_update_begin(&lp->tx_stat_sync);
>  		u64_stats_add(&lp->tx_packets, packets);
> -		u64_stats_add(&lp->tx_bytes, size);
> +		u64_stats_add(&lp->tx_bytes, lp->tx_compl_bytes);
>  		u64_stats_update_end(&lp->tx_stat_sync);
> +		lp->tx_compl_bytes = 0;
>  
>  		/* Matches barrier in axienet_start_xmit */
>  		smp_mb();
> @@ -1115,7 +1115,7 @@ axienet_start_xmit(struct sk_buff *skb, struct net_device *ndev)
>  				netdev_err(ndev, "TX DMA mapping error\n");
>  			ndev->stats.tx_dropped++;
>  			axienet_free_tx_chain(lp, orig_tail_ptr, ii + 1,
> -					      true, NULL, 0);
> +					      true, 0);
>  			dev_kfree_skb_any(skb);
>  			return NETDEV_TX_OK;
>  		}