Subject: Re: [PATCH v2 net 2/9] net: enetc: correct the tx_swbd statistics

From: Vladimir Oltean <[email-masked]>
Date: Thu, 20 Feb 2025 18:01:23 +0200

On Wed, Feb 19, 2025 at 01:42:40PM +0800, Wei Fang wrote:
> When creating a TSO header, if the skb is VLAN tagged, the extended BD
> will be used and the 'count' should be increased by 2 instead of 1.
> Otherwise, when an error occurs, less tx_swbd will be freed than the
> actual number.
> 
> Fixes: fb8629e2cbfc ("net: enetc: add support for software TSO")
> Cc: [email-masked]
> Signed-off-by: Wei Fang <[email-masked]>
> Reviewed-by: Michal Swiatkowski <[email-masked]>
> ---

I'm not sure "correct the statistics" is the best way to describe this
change. Maybe "keep track of correct TXBD count in enetc_map_tx_tso_buffs()"?
The bug is that not all TX buffers are freed on error, not that some
statistics are wrong.

>  drivers/net/ethernet/freescale/enetc/enetc.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/freescale/enetc/enetc.c b/drivers/net/ethernet/freescale/enetc/enetc.c
> index 01c09fd26f9f..0658c06a23c1 100644
> --- a/drivers/net/ethernet/freescale/enetc/enetc.c
> +++ b/drivers/net/ethernet/freescale/enetc/enetc.c
> @@ -759,6 +759,7 @@ static int enetc_lso_hw_offload(struct enetc_bdr *tx_ring, struct sk_buff *skb)
>  static int enetc_map_tx_tso_buffs(struct enetc_bdr *tx_ring, struct sk_buff *skb)
>  {
>  	struct enetc_ndev_priv *priv = netdev_priv(tx_ring->ndev);
> +	bool ext_bd = skb_vlan_tag_present(skb);
>  	int hdr_len, total_len, data_len;
>  	struct enetc_tx_swbd *tx_swbd;
>  	union enetc_tx_bd *txbd;
> @@ -792,7 +793,7 @@ static int enetc_map_tx_tso_buffs(struct enetc_bdr *tx_ring, struct sk_buff *skb
>  		csum = enetc_tso_hdr_csum(&tso, skb, hdr, hdr_len, &pos);
>  		enetc_map_tx_tso_hdr(tx_ring, skb, tx_swbd, txbd, &i, hdr_len, data_len);
>  		bd_data_num = 0;
> -		count++;
> +		count += ext_bd ? 2 : 1;
>  
>  		while (data_len > 0) {
>  			int size;
> -- 
> 2.34.1
>

stylistic nitpick: I think this implementation choice obscures the fact,
to an unfamiliar reader, that the requirement for an extended TXBD comes
from enetc_map_tx_tso_hdr(). This is because you repeat the condition
for skb_vlan_tag_present(), but it's not obvious it's correlated to the
other one. Something like the change below is more expressive in this
regard, in my opinion:

diff --git a/drivers/net/ethernet/freescale/enetc/enetc.c b/drivers/net/ethernet/freescale/enetc/enetc.c
index fe3967268a19..6178157611db 100644
--- a/drivers/net/ethernet/freescale/enetc/enetc.c
+++ b/drivers/net/ethernet/freescale/enetc/enetc.c
@@ -410,14 +410,15 @@ static int enetc_map_tx_buffs(struct enetc_bdr *tx_ring, struct sk_buff *skb)
 	return 0;
 }
 
-static void enetc_map_tx_tso_hdr(struct enetc_bdr *tx_ring, struct sk_buff *skb,
-				 struct enetc_tx_swbd *tx_swbd,
-				 union enetc_tx_bd *txbd, int *i, int hdr_len,
-				 int data_len)
+static int enetc_map_tx_tso_hdr(struct enetc_bdr *tx_ring, struct sk_buff *skb,
+				struct enetc_tx_swbd *tx_swbd,
+				union enetc_tx_bd *txbd, int *i, int hdr_len,
+				int data_len)
 {
 	union enetc_tx_bd txbd_tmp;
 	u8 flags = 0, e_flags = 0;
 	dma_addr_t addr;
+	int count = 1;
 
 	enetc_clear_tx_bd(&txbd_tmp);
 	addr = tx_ring->tso_headers_dma + *i * TSO_HEADER_SIZE;
@@ -460,7 +461,10 @@ static void enetc_map_tx_tso_hdr(struct enetc_bdr *tx_ring, struct sk_buff *skb,
 		/* Write the BD */
 		txbd_tmp.ext.e_flags = e_flags;
 		*txbd = txbd_tmp;
+		count++;
 	}
+
+	return count;
 }
 
 static int enetc_map_tx_tso_data(struct enetc_bdr *tx_ring, struct sk_buff *skb,
@@ -786,7 +790,6 @@ static int enetc_lso_hw_offload(struct enetc_bdr *tx_ring, struct sk_buff *skb)
 static int enetc_map_tx_tso_buffs(struct enetc_bdr *tx_ring, struct sk_buff *skb)
 {
 	struct enetc_ndev_priv *priv = netdev_priv(tx_ring->ndev);
-	bool ext_bd = skb_vlan_tag_present(skb);
 	int hdr_len, total_len, data_len;
 	struct enetc_tx_swbd *tx_swbd;
 	union enetc_tx_bd *txbd;
@@ -818,9 +821,9 @@ static int enetc_map_tx_tso_buffs(struct enetc_bdr *tx_ring, struct sk_buff *skb
 
 		/* compute the csum over the L4 header */
 		csum = enetc_tso_hdr_csum(&tso, skb, hdr, hdr_len, &pos);
-		enetc_map_tx_tso_hdr(tx_ring, skb, tx_swbd, txbd, &i, hdr_len, data_len);
+		count += enetc_map_tx_tso_hdr(tx_ring, skb, tx_swbd, txbd, &i,
+					      hdr_len, data_len);
 		bd_data_num = 0;
-		count += ext_bd ? 2 : 1;
 
 		while (data_len > 0) {
 			int size;