net: stmmac: Add support for XDP_TX action

This patch adds support for XDP_TX action which enables XDP program to
transmit back received frames.

This patch has been tested with the "xdp2" app located in samples/bpf
dir. The DUT receives burst traffic packet generated using pktgen script
'pktgen_sample03_burst_single_flow.sh'.

v4: Moved stmmac_tx_timer_arm() to be done once at the end of NAPI RX.
    Fixed stmmac_xdp_xmit_back() to return STMMAC_XDP_CONSUMED if
    XDP buffer to frame conversion fails. Thanks to Jakub's input.

v3: Added 'nq->trans_start = jiffies' to avoid TX time-out as we are
    sharing TX queue between slow path and XDP. Thanks to Jakub Kicinski
    for pointing out.
Signed-off-by: Ong Boon Leong <boon.leong.ong@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/stmicro/stmmac/stmmac.h

@@ -36,12 +36,18 @@ struct stmmac_resources {
 	int tx_irq[MTL_MAX_TX_QUEUES];
 };
 
+enum stmmac_txbuf_type {
+	STMMAC_TXBUF_T_SKB,
+	STMMAC_TXBUF_T_XDP_TX,
+};
+
 struct stmmac_tx_info {
 	dma_addr_t buf;
 	bool map_as_page;
 	unsigned len;
 	bool last_segment;
 	bool is_jumbo;
+	enum stmmac_txbuf_type buf_type;
 };
 
 #define STMMAC_TBS_AVAIL	BIT(0)
@@ -57,7 +63,10 @@ struct stmmac_tx_queue {
 	struct dma_extended_desc *dma_etx ____cacheline_aligned_in_smp;
 	struct dma_edesc *dma_entx;
 	struct dma_desc *dma_tx;
-	struct sk_buff **tx_skbuff;
+	union {
+		struct sk_buff **tx_skbuff;
+		struct xdp_frame **xdpf;
+	};
 	struct stmmac_tx_info *tx_skbuff_dma;
 	unsigned int cur_tx;
 	unsigned int dirty_tx;
@@ -77,6 +86,7 @@ struct stmmac_rx_buffer {
 struct stmmac_rx_queue {
 	u32 rx_count_frames;
 	u32 queue_index;
+	struct xdp_rxq_info xdp_rxq;
 	struct page_pool *page_pool;
 	struct stmmac_rx_buffer *buf_pool;
 	struct stmmac_priv *priv_data;