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Date:   Fri, 11 Jan 2019 18:22:12 +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 04:35 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).

>>> 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.

>>> 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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