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Date:   Fri, 11 Jan 2019 19:00:33 +0300
From:   Sergei Shtylyov <>
To:     Simon Horman <>
Cc:     David Miller <>,
        Magnus Damm <>,,
Subject: Re: [PATCH net] ravb: expand rx descriptor data to accommodate hw

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.

   I thought we're offloading an IP checksum? And in case of VLAN the IP datagram
starts 4 bytes further into a packet? Maybe I'm just misunderstanding things...

>>>>> This was not detected by the driver which incorrectly used the last two
>>>>> bytes of packet data as the checksum and truncate the packet by two bytes.
>>>>> This resulted all such packets being dropped.
>>>>> A work around is to disable rx checksum offload
>>>>>  # ethtool -K eth0 rx off
>>>>> This patch resolves this problem by increasing the size available for
>>>>> packet data in rx descriptors by two bytes. It also introduces
>>>>> RAVB_CSUM_LEN to make things a little clearer than "2" sprinkled lightly
>>>>> over the driver.
>>>>    What about using sizeof(__sum16) instead? That type is declared in
>>>> <linux/types.h> and used in 'struct iphdr'...
>>> As in the following?
>>> #define RAVB_CSUM_LEN sizeof(__sum16)
>>    No, as I said, I'd prefer to avoid any driver specific #define's.
> If that is your preference I can update the patch accordingly.

   Please do. :-)

>>>>> Tested on R-Car E3 (r8a77990) ES1.0 based Ebisu-4D board
>>>>> Fixes: 4d86d3818627 ("ravb: RX checksum offload")
>>>>> Signed-off-by: Simon Horman <>
>>>> [...]

MBR, Sergei

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