[PATCH net-next v3 08/14] net: ethernet: oa_tc6: Remove FCS size in RX frame

[PATCH net-next v3 08/14] net: ethernet: oa_tc6: Remove FCS size in RX frame

[Selvamani Rajagopal]

When MAC-PHY appends FCS to the incoming frame, FCS,
it is removed from the frame before passing it to the stack.

Signed-off-by: Selvamani Rajagopal <Selvamani.Rajagopal@onsemi.com>
---
 drivers/net/ethernet/oa_tc6/oa_tc6.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/net/ethernet/oa_tc6/oa_tc6.c b/drivers/net/ethernet/oa_tc6/oa_tc6.c
index d2b05f98765b..de5f1548139f 100644
--- a/drivers/net/ethernet/oa_tc6/oa_tc6.c
+++ b/drivers/net/ethernet/oa_tc6/oa_tc6.c
@@ -786,6 +786,9 @@ static void oa_tc6_submit_rx_skb(struct oa_tc6 *tc6)
 	tc6->netdev->stats.rx_packets++;
 	tc6->netdev->stats.rx_bytes += tc6->rx_skb->len;
 
+	if ((tc6->netdev->hw_features & NETIF_F_RXFCS) != 0)
+		skb_trim(tc6->rx_skb, tc6->rx_skb->len - ETH_FCS_LEN);
+
 	netif_rx(tc6->rx_skb);
 
 	tc6->rx_skb = NULL;
-- 
2.43.0

Re: [PATCH net-next v3 08/14] net: ethernet: oa_tc6: Remove FCS size in RX frame

[Andrew Lunn]

On Fri, May 29, 2026 at 06:41:00PM +0000, Selvamani Rajagopal wrote:
> When MAC-PHY appends FCS to the incoming frame, FCS,
> it is removed from the frame before passing it to the stack.

What is missing from this commit message is an answer to the question
"Why?". Does the stack do something wrong if there is a FCS at the end
of the frame?

   Andrew

RE: [PATCH net-next v3 08/14] net: ethernet: oa_tc6: Remove FCS size in RX frame

[Selvamani Rajagopal]

> 
> On Fri, May 29, 2026 at 06:41:00PM +0000, Selvamani Rajagopal wrote:
> > When MAC-PHY appends FCS to the incoming frame, FCS,
> > it is removed from the frame before passing it to the stack.
> 
> What is missing from this commit message is an answer to the question
> "Why?". Does the stack do something wrong if there is a FCS at the end
> of the frame?

That was a little mystery to me as well when our SQA alerted me with the issue.
When I tested, ping worked fine without any issue even with the frames with FCS appended. 
But strangely ptp4l utility failed with invalid data size. Once we removed FCS, it worked fine.

Will add this reason in the commit message.

> 
> Andrew

Re: [PATCH net-next v3 08/14] net: ethernet: oa_tc6: Remove FCS size in RX frame

[Qingfang Deng]

On Tue, 2 Jun 2026 at 02:40:13, Selvamani Rajagopal wrote:
> But strangely ptp4l utility failed with invalid data size. Once we removed FCS, it worked fine.

This is because ptp4l misinterprets the FCS as a TLV. There are already
several discussions and patches about this issue, however the maintainer
insists that the ethernet driver needs to be fixed instead:

https://sourceforge.net/p/linuxptp/mailman/message/37858618/
https://sourceforge.net/p/linuxptp/mailman/linuxptp-devel/thread/20190205170652.476c9676%40redhat.com/#msg36579297
https://sourceforge.net/p/linuxptp/mailman/linuxptp-devel/thread/CA%2BDf%2BjcRN98GpUi%3DF_4Ls7miEPbUZYb8O-gpSw0TQoqczVqMfw%40mail.gmail.com/#msg37858491
https://sourceforge.net/p/linuxptp/mailman/linuxptp-devel/thread/8ed1dc74-bf4c-2f13-4ed0-a70d0c4512f6%40cisco.com/#msg37137818

Regards,
Qingfang