Message-ID: <2913b19f-188e-475e-8f16-d3ad94614a08@VA3EHSMHS030.ehs.local>
Date:	Tue, 6 Apr 2010 14:12:28 -0600
From:	John Linn <John.Linn@...inx.com>
To:	<steve@...idescorp.com>, <grant.likely@...retlab.ca>
CC:	"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

> -----Original Message-----
> From: Steven J. Magnani [mailto:steve@...idescorp.com]
> Sent: Tuesday, April 06, 2010 2:04 PM
> To: grant.likely@...retlab.ca
> Cc: John Linn; Eric Dumazet; netdev@...r.kernel.org;
linuxppc-dev@...abs.org;
> jwboyer@...ux.vnet.ibm.com; john.williams@...alogix.com;
michal.simek@...alogix.com; John Tyner
> 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:
> 

Good find. I'll give it a test on MicroBlaze and PowerPC.

> /*
>  * 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?

Looks like it is defined in system.h in powerpc so that it works.

> 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?

Good idea, we can add that to system.h for MicroBlaze also.

Thanks,
John

> 
>
------------------------------------------------------------------------
>  Steven J. Magnani               "I claim this network for MARS!
>  www.digidescorp.com              Earthling, return my space
modulator!"
> 
>  #include <standard.disclaimer>
> 
> 
> 


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