[PATCH] arm64: mm: define NET_IP_ALIGN to 0

[ard.biesheuvel@linaro.org (Ard Biesheuvel)]

Hello Kunihiko,

On Wed, 28 Nov 2018 at 07:02, Kunihiko Hayashi
<hayashi.kunihiko@socionext.com> wrote:
>
> Hi Ard,
>
> I found that arm64 board that had AVE controller couldn't receive
> any packets after this change.
>
> AVE controller (hardware) forces to ignore lower 2bits of buffer start address,
> that is, the address is always aligned with 4bytes.
> And more, the controller puts received datas with "2byte headroom" [1].
>
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/net/ethernet/socionext/sni_ave.c#n591
>
> In case that NET_IP_ALIGN is 2, the driver proceeds skb->data by 2byte
> with calling netdev_alloc_skb_ip_align(), and then, skb->data is equivalent
> to start address of data reception.
>
>     +0x00 00  buffer start address
>     +0x01 00
>     +0x02 XX  skb->data == start address of data reception
>     +0x03 XX
>     ...
>
> However, in case that NET_IP_ALIGN is zero, skb->data is the same address
> as buffer start address, so skb->data differs from start address of data
> reception. Finally the driver can't receive any packets collectly.
>
>     +0x00 00  skb->data == buffer start address
>     +0x01 00
>     +0x02 XX  start address of data reception
>     +0x03 XX
>     ...
>
> Although I might be able to replace NET_IP_ALIGN with an own macro
> without calling netdev_alloc_skb_ip_align(), it seems clumsy.
> Is that any ideas?
>

If your hardware assumes NET_IP_ALIGN == 2, then yes, I suggest you
introduce your own macros here instead.