Re: [PATCH net] net: phy: mscc: Stop clearing the the UDPv4 checksum for L2 frames

[Andrew Lunn]

On Mon, May 26, 2025 at 08:54:45AM +0200, Horatiu Vultur wrote:
> The 05/23/2025 14:59, Andrew Lunn wrote:
> 
> Hi Andrew,
> 
> > 
> > On Fri, May 23, 2025 at 10:27:16AM +0200, Horatiu Vultur wrote:
> > > We have noticed that when PHY timestamping is enabled, L2 frames seems
> > > to be modified by changing two 2 bytes with a value of 0. The place were
> > > these 2 bytes seems to be random(or I couldn't find a pattern).  In most
> > > of the cases the userspace can ignore these frames but if for example
> > > those 2 bytes are in the correction field there is nothing to do.  This
> > > seems to happen when configuring the HW for IPv4 even that the flow is
> > > not enabled.
> > > These 2 bytes correspond to the UDPv4 checksum and once we don't enable
> > > clearing the checksum when using L2 frames then the frame doesn't seem
> > > to be changed anymore.
> > >
> > > Fixes: 7d272e63e0979d ("net: phy: mscc: timestamping and PHC support")
> > > Signed-off-by: Horatiu Vultur <horatiu.vultur@xxxxxxxxxxxxx>
> > > ---
> > >  drivers/net/phy/mscc/mscc_ptp.c | 4 +++-
> > >  1 file changed, 3 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/net/phy/mscc/mscc_ptp.c b/drivers/net/phy/mscc/mscc_ptp.c
> > > index 6f96f2679f0bf..6b800081eed52 100644
> > > --- a/drivers/net/phy/mscc/mscc_ptp.c
> > > +++ b/drivers/net/phy/mscc/mscc_ptp.c
> > > @@ -946,7 +946,9 @@ static int vsc85xx_ip1_conf(struct phy_device *phydev, enum ts_blk blk,
> > >       /* UDP checksum offset in IPv4 packet
> > >        * according to: https://tools.ietf.org/html/rfc768
> > >        */
> > > -     val |= IP1_NXT_PROT_UDP_CHKSUM_OFF(26) | IP1_NXT_PROT_UDP_CHKSUM_CLEAR;
> > > +     val |= IP1_NXT_PROT_UDP_CHKSUM_OFF(26);
> > > +     if (enable)
> > > +             val |= IP1_NXT_PROT_UDP_CHKSUM_CLEAR;
> > 
> > Is this towards the media, or received from the media?
> 
> It is when the vsc85xx PHY receives frames from the link partner.
> 
> >Have you tried sending packets with deliberately broken UDPv4 checksum?
> >Does the PHYs PTP engine correctly ignore such packets?
> 
> No, I have not done that. What I don't understand is why should I send
> UDPv4 frames when we enable to timestamp only L2 frames.

Ah, O.K. I was just wondering if the UDP checksumming in the hardware
is broken in general. If so, you might want to disable it, and do the
checks in software.

       Andrew