netdev - Re: [PATCH 3/3] net: fec: align IP header in hardware

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Date:   Mon, 26 Sep 2016 11:39:55 -0700
From:   Eric Nelson <eric@...int.com>
To:     David Laight <David.Laight@...LAB.COM>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Cc:     "linux@....linux.org.uk" <linux@....linux.org.uk>,
        "andrew@...n.ch" <andrew@...n.ch>,
        "fugang.duan@....com" <fugang.duan@....com>,
        "otavio@...ystems.com.br" <otavio@...ystems.com.br>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        "troy.kisky@...ndarydevices.com" <troy.kisky@...ndarydevices.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "u.kleine-koenig@...gutronix.de" <u.kleine-koenig@...gutronix.de>
Subject: Re: [PATCH 3/3] net: fec: align IP header in hardware

Hi David,

On 09/26/2016 02:26 AM, David Laight wrote:
> From: Eric Nelson
>> Sent: 24 September 2016 15:42
>> The FEC receive accelerator (RACC) supports shifting the data payload of
>> received packets by 16-bits, which aligns the payload (IP header) on a
>> 4-byte boundary, which is, if not required, at least strongly suggested
>> by the Linux networking layer.
> ...
>> +		/* align IP header */
>> +		val |= FEC_RACC_SHIFT16;
> 
> I can't help feeling that there needs to be corresponding
> changes to increase the buffer size by 2 (maybe for large mtu)
> and to discard two bytes from the frame length.
> 

In the normal case, the fec driver over-allocates all receive packets to
be of size FEC_ENET_RX_FRSIZE (2048) minus the value of rx_align,
which is either 0x0f (ARM) or 0x03 (PPC).

If the frame length is less than rx_copybreak (typically 256), then
the frame length from the receive buffer descriptor is used to
control the allocation size for a copied buffer, and this will include
the two bytes of padding if RACC_SHIFT16 is set.

> If probably ought to be predicated on NET_IP_ALIGN as well.
> 
Can you elaborate?

Please advise,


Eric