netdev - Re: [PATCH net-next] gro: introduce gro_mac_header

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Message-Id: <20120206.113145.1284864994961472499.davem@davemloft.net>
Date:	Mon, 06 Feb 2012 11:31:45 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	ogerlitz@...lanox.com, sean.hefty@...el.com,
	herbert@...dor.hengli.com.au, linux-rdma@...r.kernel.org,
	shlomop@...lanox.com, roland@...nel.org, netdev@...r.kernel.org
Subject: Re: [PATCH net-next] gro: introduce gro_mac_header_len

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Mon, 06 Feb 2012 17:27:07 +0100

> Shlomo Pongratz reported GRO L2 header check was suited for Ethernet
> only, and failed on IB/ipoib traffic.
> 
> He provided a patch faking a zeroed header to let GRO aggregates frames.
> 
> Roland Dreier, Herbert Xu, and others suggested we change GRO L2 header
> check to be more generic.
> 
> This patch introduces a new netdevice field, gro_mac_header_len, giving
> L2 header length, default to ETH_HLEN (14 bytes)
> 
> A device setup function can override this default value.
> 
> __napi_gro_receive() has special handling for the common case (Ethernet)
> to avoid a memcmp() call and use an inline optimized function instead.
> 
> Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
> Reported-by: Shlomo Pongratz <shlomop@...lanox.com>

We really need an explanation, probably both in the commit message and
the comments next to this new struct member, explaining why in the world
we can't use ->hard_header_len for this.
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