[PATCH net-next v1 3/3] net: stmmac: Optimize cache prefetch in RX path

[Furong Xu <0x1207@gmail.com>]

Current code prefetches cache lines for the received frame first, and
then dma_sync_single_for_cpu() against this frame, this is wrong.
Cache prefetch should be triggered after dma_sync_single_for_cpu().

This patch brings ~2.8% driver performance improvement in a TCP RX
throughput test with iPerf tool on a single isolated Cortex-A65 CPU
core, 2.84 Gbits/sec increased to 2.92 Gbits/sec.

Signed-off-by: Furong Xu <0x1207@gmail.com>
---
 drivers/net/ethernet/stmicro/stmmac/stmmac_main.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
index c1aeaec53b4c..1b4e8b035b1a 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
@@ -5497,10 +5497,6 @@ static int stmmac_rx(struct stmmac_priv *priv, int limit, u32 queue)
 
 		/* Buffer is good. Go on. */
 
-		prefetch(page_address(buf->page) + buf->page_offset);
-		if (buf->sec_page)
-			prefetch(page_address(buf->sec_page));
-
 		buf1_len = stmmac_rx_buf1_len(priv, p, status, len);
 		len += buf1_len;
 		buf2_len = stmmac_rx_buf2_len(priv, p, status, len);
@@ -5522,6 +5518,7 @@ static int stmmac_rx(struct stmmac_priv *priv, int limit, u32 queue)
 
 			dma_sync_single_for_cpu(priv->device, buf->addr,
 						buf1_len, dma_dir);
+			prefetch(page_address(buf->page) + buf->page_offset);
 
 			xdp_init_buff(&ctx.xdp, buf_sz, &rx_q->xdp_rxq);
 			xdp_prepare_buff(&ctx.xdp, page_address(buf->page),
@@ -5596,6 +5593,7 @@ static int stmmac_rx(struct stmmac_priv *priv, int limit, u32 queue)
 		} else if (buf1_len) {
 			dma_sync_single_for_cpu(priv->device, buf->addr,
 						buf1_len, dma_dir);
+			prefetch(page_address(buf->page) + buf->page_offset);
 			skb_add_rx_frag(skb, skb_shinfo(skb)->nr_frags,
 					buf->page, buf->page_offset, buf1_len,
 					priv->dma_conf.dma_buf_sz);
@@ -5608,6 +5606,7 @@ static int stmmac_rx(struct stmmac_priv *priv, int limit, u32 queue)
 		if (buf2_len) {
 			dma_sync_single_for_cpu(priv->device, buf->sec_addr,
 						buf2_len, dma_dir);
+			prefetch(page_address(buf->sec_page));
 			skb_add_rx_frag(skb, skb_shinfo(skb)->nr_frags,
 					buf->sec_page, 0, buf2_len,
 					priv->dma_conf.dma_buf_sz);
-- 
2.34.1