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

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Message-ID: <CAL+tcoCuvxfQUbzjSfk+7vPWLEqQgVK8muqkOQe+N6jQQwXfUw@mail.gmail.com>
Date: Thu, 8 May 2025 16:40:45 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Horatiu Vultur <horatiu.vultur@...rochip.com>
Cc: irusskikh@...vell.com, andrew+netdev@...n.ch, bharat@...lsio.com, 
	ayush.sawal@...lsio.com, UNGLinuxDriver@...rochip.com, 
	mcoquelin.stm32@...il.com, alexandre.torgue@...s.st.com, davem@...emloft.net, 
	edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com, horms@...nel.org, 
	sgoutham@...vell.com, willemb@...gle.com, 
	linux-stm32@...md-mailman.stormreply.com, 
	linux-arm-kernel@...ts.infradead.org, netdev@...r.kernel.org, 
	Jason Xing <kernelxing@...cent.com>
Subject: Re: [PATCH net-next v1 4/4] net: lan966x: generate software timestamp
 just before the doorbell

Hi Horatiu,

On Thu, May 8, 2025 at 3:08 PM Horatiu Vultur
<horatiu.vultur@...rochip.com> wrote:
>
> The 05/08/2025 11:33, Jason Xing wrote:
> >
> > From: Jason Xing <kernelxing@...cent.com>
> >
> > Make sure the call of skb_tx_timestamp as close to the doorbell.
> >
> > Signed-off-by: Jason Xing <kernelxing@...cent.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@...rochip.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