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

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Message-ID: <3a5c3a17-a410-fba7-32eb-a3495c5378de@cogentembedded.com>
Date:   Thu, 10 Jan 2019 18:52:51 +0300
From:   Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
To:     Simon Horman <horms+renesas@...ge.net.au>,
        David Miller <davem@...emloft.net>
Cc:     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 01/10/2019 05:02 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?

> 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'...

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

MBR, Sergei