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

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Message-ID: <20250508094156.kbegdd5vianotsrr@DEN-DL-M31836.microchip.com>
Date: Thu, 8 May 2025 11:41:56 +0200
From: Horatiu Vultur <horatiu.vultur@...rochip.com>
To: Jason Xing <kerneljasonxing@...il.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

The 05/08/2025 16:40, Jason Xing wrote:
> Hi Horatiu,

Hi Jason,

> 
> 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?

Well, the bpf program will check if it is a PTP frame that needs to be
timestamp when it runs ptp_classify_raw, and as we push some data in
front of the frame, the bpf will run from that point meaning that it
would failed to detect the PTP frames.

> May I ask
> how the modified data of skb breaks the PHY timestamping feature?

If it fails to detect that it is a PTP frame, then the frame will not be
passed to the PHY using the callback txtstamp. So the PHY will timestamp the
frame but it doesn't have the frame to attach the timestamp.

> 
> 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

-- 
/Horatiu