net: stmmac: vlan: Prevent redundant desc allocation for VLAN tags

Both DW GMAC4 and DW XGMAC HW-manuals claim that the VLAN tags specified
via the context descriptors are stored in the DMA-engine and re-used on
the next packet marked as VLAN tagged:

> The VLAN Tag IDs and MSS values, provided by the application in a
> context descriptor with their corresponding Valid bits set, are stored
> internally by the DMA. When the outer or inner VLAN tag is provided
> with the Valid bit set, the DMA always passes the last valid VLAN tag to
> the MTL. The application cannot invalidate the valid VLAN tag stored by
> the DMA. The VLAN tag is inserted or replaced based on the control
> inputs provided for the packet.

Thus it's redundant to allocate the context DMA-descriptor each time a
VLAN tagged packet is transmitted in case if it' tag has already been
stored by the DMA-engine of the respective queue. Let's cache the VLAN tag
then and skip the allocation if a frame with the same tag is specified.
This shall improve the xfer performance and reduce the DMA-descriptors
consumption rate.

Signed-off-by: Serge Semin <fancer.lancer@gmail.com>

2 files changed, 10 insertions, 0 deletions

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac.h b/drivers/net/ethernet/stmicro/stmmac/stmmac.h
index b1188d897936..7ac58e01d846 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac.h
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac.h
@@ -119,6 +119,7 @@ struct stmmac_tx_queue {
 	dma_addr_t dma_tx_phy;
 	dma_addr_t tx_tail_addr;
 	u32 mss;
+	u16 tci;
 	u16 ttc;
 };
 
diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
index 1a6d2f6335b5..9dad81d3ffea 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
@@ -4406,6 +4406,13 @@ static bool stmmac_vlan_insert(struct stmmac_priv *priv, struct sk_buff *skb,
 
 	tag = skb_vlan_tag_get(skb);
 
+	/* The previously set outer VLAN Tag is stored in the DMA-engine.
+	 * Skip setting it again if it is the same is in the previous xfers.
+	 * Note TPID is selected by the activated Tx CTAG/STAG feature.
+	 */
+	if (tx_q->tci == tag)
+		return true;
+
 	if (tx_q->tbs & STMMAC_TBS_AVAIL)
 		p = &tx_q->dma_entx[tx_q->cur_tx].basic;
 	else
@@ -4416,6 +4423,7 @@ static bool stmmac_vlan_insert(struct stmmac_priv *priv, struct sk_buff *skb,
 
 	stmmac_set_tx_owner(priv, p);
 	tx_q->cur_tx = STMMAC_GET_ENTRY(tx_q->cur_tx, priv->dma_conf.dma_tx_size);
+	tx_q->tci = tag;
 	return true;
 }
 
@@ -9025,6 +9033,7 @@ static void stmmac_reset_tx_queue(struct stmmac_priv *priv, u32 queue)
 	tx_q->cur_tx = 0;
 	tx_q->dirty_tx = 0;
 	tx_q->mss = 0;
+	tx_q->tci = 0;
 
 	netdev_tx_reset_queue(netdev_get_tx_queue(priv->dev, queue));
 }