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

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Date:	Tue, 6 Apr 2010 10:12:05 -0600
From:	John Linn <John.Linn@...inx.com>
To:	"Eric Dumazet" <eric.dumazet@...il.com>
CC:	<netdev@...r.kernel.org>, <linuxppc-dev@...abs.org>,
	<grant.likely@...retlab.ca>, <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: Eric Dumazet [mailto:eric.dumazet@...il.com]
> Sent: Monday, April 05, 2010 3:30 PM
> To: John Linn
> Cc: netdev@...r.kernel.org; linuxppc-dev@...abs.org; grant.likely@...retlab.ca;
> 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
> 
> Le lundi 05 avril 2010 à 15:11 -0600, John Linn a écrit :
> > This patch adds support for using the LL TEMAC Ethernet driver on
> > non-Virtex 5 platforms by adding support for accessing the Soft DMA
> > registers as if they were memory mapped instead of solely through the
> > DCR's (available on the Virtex 5).
> >
> > The patch also updates the driver so that it runs on the MicroBlaze.
> > The changes were tested on the PowerPC 440, PowerPC 405, and the
> > MicroBlaze platforms.
> >
> > Signed-off-by: John Tyner <jtyner@...ucr.edu>
> > Signed-off-by: John Linn <john.linn@...inx.com>
> >
> > ---
> 
> > +/* Align the IP data in the packet on word boundaries as MicroBlaze
> > + * needs it.
> > + */
> > +
> >  #define XTE_ALIGN       32
> > -#define BUFFER_ALIGN(adr) ((XTE_ALIGN - ((u32) adr)) % XTE_ALIGN)
> > +#define BUFFER_ALIGN(adr) ((34 - ((u32) adr)) % XTE_ALIGN)
> >
> 
> Very interesting way of doing this, but why such convoluted thing ?

This is trying to align for a cache line (32 bytes) before my change.

My change was then also making it align the IP data on a word boundary. 

> 
> Because of the % 32, this is equivalent to :
> 
> #define BUFFER_ALIGN(adr) ((2 - ((u32) adr)) % XTE_ALIGN)
> 

Yes, but I'm not sure that's clearer IMHO.

> But wait, dont we recognise the magic constant NET_IP_ALIGN ?

Yes it could be used.  I'm struggling with how to make this all be clearer.

How about this?

#define BUFFER_ALIGN(adr) (((XTE_ALIGN + NET_IP_ALIGN) - ((u32) adr)) % XTE_ALIGN)

> 
> So, I ask, cant you use netdev_alloc_skb_ip_align() in this driver ?

From what I can tell, this wouldn't work as it only reserves the 2 bytes to align with 
a word boundary.

Thanks,
John

> 
> 


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