net: enetc: add a dedicated is_eof bit in the TX software BD

In the transmit path, if we have a scatter/gather frame, it is put into
multiple software buffer descriptors, the last of which has the skb
pointer populated (which is necessary for rearming the TX MSI vector and
for collecting the two-step TX timestamp from the TX confirmation path).

At the moment, this is sufficient, but with XDP_TX, we'll need to
service TX software buffer descriptors that don't have an skb pointer,
however they might be final nonetheless. So add a dedicated bit for
final software BDs that we populate and check explicitly. Also, we keep
looking just for an skb when doing TX timestamping, because we don't
want/need that for XDP.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/freescale/enetc/enetc.c

@@ -157,6 +157,7 @@ static int enetc_map_tx_buffs(struct enetc_bdr *tx_ring, struct sk_buff *skb,
 	temp_bd.flags = flags;
 	*txbd = temp_bd;
 
+	tx_ring->tx_swbd[i].is_eof = true;
 	tx_ring->tx_swbd[i].skb = skb;
 
 	enetc_bdr_idx_inc(tx_ring, &i);
@@ -316,8 +317,6 @@ static bool enetc_clean_tx_ring(struct enetc_bdr *tx_ring, int napi_budget)
 	do_tstamp = false;
 
 	while (bds_to_clean && tx_frm_cnt < ENETC_DEFAULT_TX_WORK) {
-		bool is_eof = !!tx_swbd->skb;
-
 		if (unlikely(tx_swbd->check_wb)) {
 			struct enetc_ndev_priv *priv = netdev_priv(ndev);
 			union enetc_tx_bd *txbd;
@@ -335,7 +334,7 @@ static bool enetc_clean_tx_ring(struct enetc_bdr *tx_ring, int napi_budget)
 		if (likely(tx_swbd->dma))
 			enetc_unmap_tx_buff(tx_ring, tx_swbd);
 
-		if (is_eof) {
+		if (tx_swbd->skb) {
 			if (unlikely(do_tstamp)) {
 				enetc_tstamp_tx(tx_swbd->skb, tstamp);
 				do_tstamp = false;
@@ -355,7 +354,7 @@ static bool enetc_clean_tx_ring(struct enetc_bdr *tx_ring, int napi_budget)
 		}
 
 		/* BD iteration loop end */
-		if (is_eof) {
+		if (tx_swbd->is_eof) {
 			tx_frm_cnt++;
 			/* re-arm interrupt source */
 			enetc_wr_reg_hot(tx_ring->idr, BIT(tx_ring->index) |

drivers/net/ethernet/freescale/enetc/enetc.h

@@ -25,6 +25,7 @@ struct enetc_tx_swbd {
 	u8 is_dma_page:1;
 	u8 check_wb:1;
 	u8 do_tstamp:1;
+	u8 is_eof:1;
 };
 
 #define ENETC_RX_MAXFRM_SIZE	ENETC_MAC_MAXFRM_SIZE