netdev - Re: [PATCH net] ravb: expand rx descriptor data to accommodate hw checksum

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Message-ID: <bc344c4a-6f4e-ca65-e7d2-977f91f2662c@cogentembedded.com>
Date:   Fri, 18 Jan 2019 17:58:29 +0300
From:   Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
To:     Simon Horman <horms@...ge.net.au>
Cc:     David Miller <davem@...emloft.net>,
        Magnus Damm <magnus.damm@...il.com>, netdev@...r.kernel.org,
        linux-renesas-soc@...r.kernel.org
Subject: Re: [PATCH net] ravb: expand rx descriptor data to accommodate hw
 checksum

Hello!

On 01/11/2019 06:48 PM, Simon Horman wrote:

>>>>> EtherAVB may provide a checksum of packet data appended to packet data. In
>>>>> order to allow this checksum to be received by the host descriptor data
>>>>> needs to be enlarged by 2 bytes to accommodate the checksum.
>>>>>
>>>>> In the case of MTU-sized packets without a VLAN tag the
>>>>> checksum were already accommodated by virtue of the space reserved for the
>>>>> VLAN tag. However, a packet of MTU-size with a  VLAN tag consumed all
>>>>> packet data space provided by a descriptor leaving no space for the
>>>>> trailing checksum.
>>>>
>>>>    Wait! The gen3 manual is rather clear about the auto-checksumming not working
>>>> right in the presence of the VLAN tag. Where do you check for that case?
>>>
>>> In my testing on E3 this works correctly. Which portion of
>>> the manual are you referring to?
>>
>>    Section 50.4.1 in R-Car Series, 3rd Generation User’s Manual: Hardware, Rev 1.00,
>> section 45A.3.14.1 in R-Car Series, 2nd Generation User’s Manual: Hardware, Rev 2.00.
>>    The problem is that the checksum is always calculated starting at byte
>> 14, i.e.  amidst the VLAN header (if present).
> 
> Yes, I understand that. What I'm unclear on is why that is a problem.
> Empirically I have observed that RX csum offload works in the presence
> of a VLAN tag and offers a significant benefit in terms of reduced CPU
> utilisation.

   OK, after having looked a the networking code, I agree there shouldn't be issues
with VLAN packets.

[...]

MBR, Sergei