[PATCH net-next v1 4/4] net: lan966x: generate software timestamp just before the doorbell

[Jason Xing]

Hi Horatiu,

On Thu, May 8, 2025 at 3:08 PM Horatiu Vultur
<horatiu.vultur at microchip.com> wrote:
>
> The 05/08/2025 11:33, Jason Xing wrote:
> >
> > From: Jason Xing <kernelxing at tencent.com>
> >
> > Make sure the call of skb_tx_timestamp as close to the doorbell.
> >
> > Signed-off-by: Jason Xing <kernelxing at tencent.com>
> > ---
> >  drivers/net/ethernet/microchip/lan966x/lan966x_fdma.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/net/ethernet/microchip/lan966x/lan966x_fdma.c b/drivers/net/ethernet/microchip/lan966x/lan966x_fdma.c
> > index 502670718104..e030f23e5145 100644
> > --- a/drivers/net/ethernet/microchip/lan966x/lan966x_fdma.c
> > +++ b/drivers/net/ethernet/microchip/lan966x/lan966x_fdma.c
> > @@ -730,7 +730,6 @@ int lan966x_fdma_xmit(struct sk_buff *skb, __be32 *ifh, struct net_device *dev)
> >                 }
> >         }
> >
> > -       skb_tx_timestamp(skb);
>
> Changing this will break the PHY timestamping because the frame gets
> modified in the next line, meaning that the classify function will
> always return PTP_CLASS_NONE.

Sorry that I'm not that familiar with the details. I will remove it
from this series, but still trying to figure out what cases could be.

Do you mean it can break when bpf prog is loaded because
'skb_push(skb, IFH_LEN_BYTES);' expands the skb->data area?  May I ask
how the modified data of skb breaks the PHY timestamping feature?

Thanks,
Jason

>
> Nacked-by: Horatiu Vultur <horatiu.vultur at microchip.com>
>
> >         skb_push(skb, IFH_LEN_BYTES);
> >         memcpy(skb->data, ifh, IFH_LEN_BYTES);
> >         skb_put(skb, 4);
> > @@ -768,6 +767,7 @@ int lan966x_fdma_xmit(struct sk_buff *skb, __be32 *ifh, struct net_device *dev)
> >                 next_dcb_buf->ptp = true;
> >
> >         /* Start the transmission */
> > +       skb_tx_timestamp(skb);
> >         lan966x_fdma_tx_start(tx);
> >
> >         return NETDEV_TX_OK;
> > --
> > 2.43.5
> >
>
> --
> /Horatiu