axienet_free_tx_chain returns the number of DMA descriptors it's
handled. However, axienet_tx_poll treats the return as the number of
packets. When scatter-gather SKBs are enabled, a single packet may use
multiple DMA descriptors, which causes incorrect packet counts. Fix this
by explicitly keepting track of the number of packets processed as
separate from the DMA descriptors.

Fixes: 8a3b7a252dca ("drivers/net/ethernet/xilinx: added Xilinx AXI Ethernet driver")
Signed-off-by: Sean Anderson <sean.anderson@linux.dev>
---

 .../net/ethernet/xilinx/xilinx_axienet_main.c | 29 +++++++++++--------
 1 file changed, 17 insertions(+), 12 deletions(-)

diff --git a/drivers/net/ethernet/xilinx/xilinx_axienet_main.c b/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
index 9aeb7b9f3ae4..86da628f66fd 100644
--- a/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
+++ b/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
@@ -670,21 +670,21 @@ static int axienet_device_reset(struct net_device *ndev)
  * @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)
+ * @budget:	NAPI budget (use INT_MAX when not called from NAPI poll)
  *
  * Would either be called after a successful transmit operation, or after
  * there was an error when setting up the chain.
- * Returns the number of descriptors handled.
+ * Returns 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)
 {
 	struct axidma_bd *cur_p;
 	unsigned int status;
+	int i, packets = 0;
 	dma_addr_t phys;
-	int i;
 
-	for (i = 0; i < nr_bds; i++) {
+	for (i = 0; i < nr_bds && packets < budget; i++) {
 		cur_p = &lp->tx_bd_v[(first_bd + i) % lp->tx_bd_num];
 		status = cur_p->status;
 
@@ -701,8 +701,10 @@ static int axienet_free_tx_chain(struct axienet_local *lp, u32 first_bd,
 				 (cur_p->cntrl & XAXIDMA_BD_CTRL_LENGTH_MASK),
 				 DMA_TO_DEVICE);
 
-		if (cur_p->skb && (status & XAXIDMA_BD_STS_COMPLETE_MASK))
+		if (cur_p->skb && (status & XAXIDMA_BD_STS_COMPLETE_MASK)) {
 			napi_consume_skb(cur_p->skb, budget);
+			packets++;
+		}
 
 		cur_p->app0 = 0;
 		cur_p->app1 = 0;
@@ -718,7 +720,13 @@ static int axienet_free_tx_chain(struct axienet_local *lp, u32 first_bd,
 			*sizep += status & XAXIDMA_BD_STS_ACTUAL_LEN_MASK;
 	}
 
-	return i;
+	if (!force) {
+		lp->tx_bd_ci += i;
+		if (lp->tx_bd_ci >= lp->tx_bd_num)
+			lp->tx_bd_ci %= lp->tx_bd_num;
+	}
+
+	return packets;
 }
 
 /**
@@ -891,13 +899,10 @@ static int axienet_tx_poll(struct napi_struct *napi, int budget)
 	u32 size = 0;
 	int packets;
 
-	packets = axienet_free_tx_chain(lp, lp->tx_bd_ci, budget, false, &size, budget);
+	packets = axienet_free_tx_chain(lp, lp->tx_bd_ci, lp->tx_bd_num, false,
+					&size, budget);
 
 	if (packets) {
-		lp->tx_bd_ci += packets;
-		if (lp->tx_bd_ci >= lp->tx_bd_num)
-			lp->tx_bd_ci %= lp->tx_bd_num;
-
 		u64_stats_update_begin(&lp->tx_stat_sync);
 		u64_stats_add(&lp->tx_packets, packets);
 		u64_stats_add(&lp->tx_bytes, size);
@@ -1003,7 +1008,7 @@ 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, NULL, INT_MAX);
 			return NETDEV_TX_OK;
 		}
 		desc_set_phys_addr(lp, phys, cur_p);
-- 
2.35.1.1320.gc452695387.dirty

On Tue,  3 Sep 2024 13:56:19 -0400 Sean Anderson wrote:
> axienet_free_tx_chain returns the number of DMA descriptors it's
> handled. However, axienet_tx_poll treats the return as the number of
> packets. When scatter-gather SKBs are enabled, a single packet may use
> multiple DMA descriptors, which causes incorrect packet counts. Fix this
> by explicitly keepting track of the number of packets processed as
> separate from the DMA descriptors.

budget doubles up as a "are we really in NAPI" flag.
You can't pass non-zero budget to napi_consume_skb() if not in NAPI.
-- 
pw-bot: cr

On 9/4/24 20:09, Jakub Kicinski wrote:
> On Tue,  3 Sep 2024 13:56:19 -0400 Sean Anderson wrote:
>> axienet_free_tx_chain returns the number of DMA descriptors it's
>> handled. However, axienet_tx_poll treats the return as the number of
>> packets. When scatter-gather SKBs are enabled, a single packet may use
>> multiple DMA descriptors, which causes incorrect packet counts. Fix this
>> by explicitly keepting track of the number of packets processed as
>> separate from the DMA descriptors.
> 
> budget doubles up as a "are we really in NAPI" flag.
> You can't pass non-zero budget to napi_consume_skb() if not in NAPI.

Hm, maybe I should just determine this all from the "force" flag then.

--Sean