netdev - [PATCH] net: igb: Only dma sync frame length

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Message-Id: <1464975932-13301-1-git-send-email-andrew@lunn.ch>
Date:	Fri,  3 Jun 2016 19:45:32 +0200
From:	Andrew Lunn <andrew@...n.ch>
To:	Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
	David Miller <davem@...emloft.net>
Cc:	netdev <netdev@...r.kernel.org>, intel-wired-lan@...ts.osuosl.org,
	Andrew Lunn <andrew@...n.ch>
Subject: [PATCH] net: igb: Only dma sync frame length

On some platforms, syncing a buffer for DMA is expensive. Rather than
sync the whole 2K receive buffer, only synchronise the length of the
frame, which will typically be the MTU, or a much smaller TCP ACK.

For an IMX6Q, this gives around 6% increased TCP receive performance,
which is cache operations bound and reduces CPU load for TCP transmit.

Signed-off-by: Andrew Lunn <andrew@...n.ch>
---
 drivers/net/ethernet/intel/igb/igb_main.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/intel/igb/igb_main.c b/drivers/net/ethernet/intel/igb/igb_main.c
index 0a289dda604a..670e3d612283 100644
--- a/drivers/net/ethernet/intel/igb/igb_main.c
+++ b/drivers/net/ethernet/intel/igb/igb_main.c
@@ -6915,6 +6915,7 @@ static struct sk_buff *igb_fetch_rx_buffer(struct igb_ring *rx_ring,
 {
 	struct igb_rx_buffer *rx_buffer;
 	struct page *page;
+	unsigned int size = le16_to_cpu(rx_desc->wb.upper.length);
 
 	rx_buffer = &rx_ring->rx_buffer_info[rx_ring->next_to_clean];
 	page = rx_buffer->page;
@@ -6948,7 +6949,7 @@ static struct sk_buff *igb_fetch_rx_buffer(struct igb_ring *rx_ring,
 	dma_sync_single_range_for_cpu(rx_ring->dev,
 				      rx_buffer->dma,
 				      rx_buffer->page_offset,
-				      IGB_RX_BUFSZ,
+				      size,
 				      DMA_FROM_DEVICE);
 
 	/* pull page into skb */
-- 
2.8.1