netdev - Re: [PATCH v2 4/9] mlxsw: spectrum_ptp: Use generic helper function

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Date:   Wed, 29 Jul 2020 16:07:14 +0200
From:   Petr Machata <petrm@...lanox.com>
To:     Kurt Kanzenbach <kurt@...utronix.de>
Cc:     Russell King - ARM Linux admin <linux@...linux.org.uk>,
        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>,
        Grygorii Strashko <grygorii.strashko@...com>,
        Ivan Khoronzhuk <ivan.khoronzhuk@...aro.org>,
        Samuel Zou <zou_wei@...wei.com>, netdev@...r.kernel.org
Subject: Re: [PATCH v2 4/9] mlxsw: spectrum_ptp: Use generic helper function


Kurt Kanzenbach <kurt@...utronix.de> writes:

> On Wed Jul 29 2020, Russell King - ARM Linux admin wrote:
>> Would it make more sense to do:
>>
>> 	u8 *data = skb_mac_header(skb);
>> 	u8 *ptr = data;
>>
>> 	if (type & PTP_CLASS_VLAN)
>> 		ptr += VLAN_HLEN;
>>
>> 	switch (type & PTP_CLASS_PMASK) {
>> 	case PTP_CLASS_IPV4:
>> 		ptr += IPV4_HLEN(ptr) + UDP_HLEN;
>> 		break;
>>
>> 	case PTP_CLASS_IPV6:
>> 		ptr += IP6_HLEN + UDP_HLEN;
>> 		break;
>>
>> 	case PTP_CLASS_L2:
>> 		break;
>>
>> 	default:
>> 		return NULL;
>> 	}
>>
>> 	ptr += ETH_HLEN;
>>
>> 	if (ptr + 34 > skb->data + skb->len)
>> 		return NULL;
>>
>> 	return ptr;
>>
>> in other words, compare pointers, so that whether skb_push() etc has
>> been used on the skb is irrelevant?

I like this!

> The ptp v1 code always does locate the message type at
>
>  msgtype = data + offset + OFF_PTP_CONTROL
>
> OFF_PTP_CONTROL is 32. However, looking at the ptp v1 header, the
> message type is located at offset 20. What am I missing here?

0x20 == 32? I see it at offset 32 in IEEE 1588.