netdev - Re: RE packet: fix reserve calculation

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Date:   Fri, 7 Sep 2018 23:18:04 -0400
From:   James Sakalaukus <james@...alaukus.com>
To:     David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Cc:     Willem de Bruijn <willemb@...gle.com>
Subject: Re: RE packet: fix reserve calculation - net/packet/af_packet.c

On Fri, Sep 7, 2018 at 1:34 PM, David Miller <davem@...emloft.net> wrote:
> From: Willem de Bruijn <willemb@...gle.com>
> Date: Fri, 7 Sep 2018 11:50:02 -0400
>
>> If you are not seeing these problems with other protocols, I must be
>> misreading that code.
>
> Right, the ethernet header is only guaranteed to be 2 byte aligned on
> transmit.
>
> Nothing ever gave larger alignment guarantees.
>
> This is the _only_ reasonable situation.
>
> If we made the ethernet header to be 4 byte aligned, that means the
> ipv4 addresses in the ipv4 header are not 4 byte aligned so we would
> do unaligned memory accesses when building the headers which are
> either expensive or trap depending upon the architecture.

Understood.

I thought SOCK_RAW made no assumptions with respect to the network
layer headers.  Previously, the buffer was allocated and then data was
copied into it without giving thought to alignment.  Sounds like I got
lucky with the alignment and I should have expected 2-bytes from day
1.

I've been using this device with only SOCK_RAW on a real time network,
and I have not even tried to use SOCK_STREAM or SOCK_DGRAM.
So, I just tried to use TCP on the device, and the buffer alignment is
in fact 2-bytes inside my transmit function, and my DMA core does not
work.  So...I guess I need to bring my hardware into the 21st century
or do a copy in the driver transmit function.

Thanks for your time