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

From: Russell King - ARM Linux admin <[email-masked]>
Date: Tue, 4 Aug 2020 22:07:59 +0100

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 <[email-masked]> 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/
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