netdev - RE: pch_gbe: dont-copy-payload (was [PATCH 1/4] ...)

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Message-ID: <1344264756.26674.19.camel@edumazet-glaptop>
Date:	Mon, 06 Aug 2012 16:52:36 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Andy Cress <andy.cress@...kontron.com>
Cc:	netdev@...r.kernel.org
Subject: RE: pch_gbe: dont-copy-payload (was [PATCH 1/4] ...)

On Mon, 2012-08-06 at 07:19 -0700, Andy Cress wrote:
> I found out why the proposed dont-copy-payload patch didn't work with
> this pch_gbe NIC.
> This NIC PHY requires 64-byte-aligned DMA, and the transmit buffers
> won't be transferred if skb->data is not 64-byte-aligned.  Apparently
> the data copy has a by-product of aligning the buffers.  
> 
> I tried using skb_reserve(skb,64) in pch_gbe_alloc_tx_buffers and
> pch-gbe_alloc_rx_buffers, but that didn't seem to resolve it.
> 
> How can I make sure that the transmit data buffers are
> 64-byte-aligned?
> 

There is no support for such requirement in linux stacks.

Only solution for the driver is to copy all frames to 64-byte-aligned
bounce buffers.



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