netdev - Re: [PATCH v3 1/9] ptp: Add generic ptp v2 header parsing function

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Message-ID: <20200804210759.GU1551@shell.armlinux.org.uk>
Date:   Tue, 4 Aug 2020 22:07:59 +0100
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Grygorii Strashko <grygorii.strashko@...com>
Cc:     Kurt Kanzenbach <kurt@...utronix.de>,
        Petr Machata <petrm@...lanox.com>,
        Richard Cochran <richardcochran@...il.com>,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jiri Pirko <jiri@...lanox.com>,
        Ido Schimmel <idosch@...lanox.com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Ivan Khoronzhuk <ivan.khoronzhuk@...aro.org>,
        Samuel Zou <zou_wei@...wei.com>, netdev@...r.kernel.org
Subject: Re: [PATCH v3 1/9] ptp: Add generic ptp v2 header parsing function

On Tue, Aug 04, 2020 at 11:56:12PM +0300, Grygorii Strashko wrote:
> 
> 
> On 31/07/2020 13:06, Kurt Kanzenbach wrote:
> > On Thu Jul 30 2020, Petr Machata wrote:
> > > Kurt Kanzenbach <kurt@...utronix.de> writes:
> > > 
> > > > @@ -107,6 +107,37 @@ unsigned int ptp_classify_raw(const struct sk_buff *skb)
> > > >   }
> > > >   EXPORT_SYMBOL_GPL(ptp_classify_raw);
> > > > +struct ptp_header *ptp_parse_header(struct sk_buff *skb, unsigned int type)
> > > > +{
> > > > +	u8 *data = skb_mac_header(skb);
> > > > +	u8 *ptr = data;
> > > 
> > > One of the "data" and "ptr" variables is superfluous.
> > 
> > Yeah. Can be shortened to u8 *ptr = skb_mac_header(skb);
> 
> Actually usage of skb_mac_header(skb) breaks CPTS RX time-stamping on
> am571x platform PATCH 6.
> 
> The CPSW RX timestamp requested after full packet put in SKB, but
> before calling eth_type_trans().
> 
> So, skb->data pints on Eth header, but skb_mac_header() return garbage.
> 
> Below diff fixes it for me.

However, that's likely to break everyone else.

For example, anyone calling this from the mii_timestamper rxtstamp()
method, the skb will have been classified with the MAC header pushed
and restored, so skb->data points at the network header.

Your change means that ptp_parse_header() expects the MAC header to
also be pushed.

Is it possible to adjust CPTS?

Looking at:
drivers/net/ethernet/ti/cpsw.c... yes.
drivers/net/ethernet/ti/cpsw_new.c... yes.
drivers/net/ethernet/ti/netcp_core.c... unclear.

If not, maybe cpts should remain unconverted - I don't see any reason
to provide a generic function for one user.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!