netdev - Re: [PATCH] [V3] Add non-Virtex5 support for LL TEMAC driver

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Message-Id: <1270584223.3949.57.camel@iscandar.digidescorp.com>
Date:	Tue, 06 Apr 2010 15:03:43 -0500
From:	"Steven J. Magnani" <steve@...idescorp.com>
To:	Grant Likely <grant.likely@...retlab.ca>
Cc:	John Linn <John.Linn@...inx.com>,
	Eric Dumazet <eric.dumazet@...il.com>, netdev@...r.kernel.org,
	linuxppc-dev@...abs.org, jwboyer@...ux.vnet.ibm.com,
	john.williams@...alogix.com, michal.simek@...alogix.com,
	John Tyner <jtyner@...ucr.edu>
Subject: Re: [PATCH] [V3] Add non-Virtex5 support for LL TEMAC driver

On Tue, 2010-04-06 at 12:53 -0600, Grant Likely wrote:

> Hold on.... BUFFER_ALIGN is being used to align the DMA buffer on a
> cache line boundary.  I don't think netdev_alloc_skb() makes any
> guarantees about how the start of the IP header lines up against cache
> line boundaries.  The amount of padding needed is not known until an
> skbuff is obtained from netdev_alloc_skb(), and
> netdev_alloc_skb_ip_align() can only handle a fixed size padding,
> 
> It doesn't look like netdev_alloc_skb_ip_align() is the right thing in
> this regard.

__netdev_alloc_skb reserves NET_SKB_PAD bytes which gets us cacheline
alignment on Microblaze. From include/linux/skbuff.h:

/*
 * The networking layer reserves some headroom in skb data (via
 * dev_alloc_skb). This is used to avoid having to reallocate skb data
when
 * the header has to grow. In the default case, if the header has to
grow
 * 32 bytes or less we avoid the reallocation.
 *
 * Unfortunately this headroom changes the DMA alignment of the
resulting
 * network packet. As for NET_IP_ALIGN, this unaligned DMA is expensive
 * on some architectures. An architecture can override this value,
 * perhaps setting it to a cacheline in size (since that will maintain
 * cacheline alignment of the DMA). It must be a power of 2.
 *
 * Various parts of the networking layer expect at least 32 bytes of
 * headroom, you should not reduce this.
 */
#ifndef NET_SKB_PAD
#define NET_SKB_PAD     32
#endif

If this doesn't work for some of the PPC variants with larger cache
lines, maybe one of the PPC header files needs to define NET_SKB_PAD?
And if we want to guard against possible future changes to the default
NET_SKB_PAD breaking Microblaze operation, maybe one of its headers
should define NET_SKB_PAD as well?

------------------------------------------------------------------------
 Steven J. Magnani               "I claim this network for MARS!
 www.digidescorp.com              Earthling, return my space modulator!"

 #include <standard.disclaimer>

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