[PATCH net] net: mscc: fix the frame extraction into the skb

[Antoine Tenart <antoine.tenart@bootlin.com>]

When extracting frames from the Ocelot switch, the frame check sequence
(FCS) is present at the end of the data extracted. The FCS was put into
the sk buffer which introduced some issues (as length related ones), as
the FCS shouldn't be part of an Rx sk buffer.

This patch fixes the Ocelot switch extraction behaviour by discarding
the FCS.

Fixes: a556c76adc05 ("net: mscc: Add initial Ocelot switch support")
Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
---

Hi all,

Once this patch will be accepted and net merged into net-next, I'll
probably send another patch to fill skb->csum with the FCS based on
NETIF_F_RXFCS.

Thanks!
Antoine

 drivers/net/ethernet/mscc/ocelot_board.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/drivers/net/ethernet/mscc/ocelot_board.c b/drivers/net/ethernet/mscc/ocelot_board.c
index 26bb3b18f3be..3cdf63e35b53 100644
--- a/drivers/net/ethernet/mscc/ocelot_board.c
+++ b/drivers/net/ethernet/mscc/ocelot_board.c
@@ -91,7 +91,7 @@ static irqreturn_t ocelot_xtr_irq_handler(int irq, void *arg)
 		struct sk_buff *skb;
 		struct net_device *dev;
 		u32 *buf;
-		int sz, len;
+		int sz, len, buf_len;
 		u32 ifh[4];
 		u32 val;
 		struct frame_info info;
@@ -116,14 +116,20 @@ static irqreturn_t ocelot_xtr_irq_handler(int irq, void *arg)
 			err = -ENOMEM;
 			break;
 		}
-		buf = (u32 *)skb_put(skb, info.len);
+		buf_len = info.len - ETH_FCS_LEN;
+		buf = (u32 *)skb_put(skb, buf_len);
 
 		len = 0;
 		do {
 			sz = ocelot_rx_frame_word(ocelot, grp, false, &val);
 			*buf++ = val;
 			len += sz;
-		} while ((sz == 4) && (len < info.len));
+		} while (len < buf_len);
+
+		/* Read the FCS and discard it */
+		sz = ocelot_rx_frame_word(ocelot, grp, false, &val);
+		/* Update the statistics if part of the FCS was read before */
+		len -= ETH_FCS_LEN - sz;
 
 		if (sz < 0) {
 			err = sz;
-- 
2.17.1