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

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Message-ID: <bf8eac92-5008-4c1f-9b99-0199e62436fa@SG2EHSMHS010.ehs.local>
Date:	Tue, 6 Apr 2010 11:11:53 -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: Tuesday, April 06, 2010 11:01 AM
> 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 mardi 06 avril 2010 à 10:12 -0600, John Linn a écrit :
> > > -----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.
> >
> 
> I am not saying its clearer, I am saying we have a standard way to
> handle this exact problem (aligning rcvs buffer so that IP header is
> aligned)
> 
> There is no need to invent new ones, this makes reviewing of this driver
> more difficult.
> 
> 
> > How about this?
> > #define BUFFER_ALIGN(adr) (((XTE_ALIGN + NET_IP_ALIGN) - ((u32) adr)) % XTE_ALIGN)
> >
> 
> Sorry, I still dont understand why you need XTE_ALIGN + ...
> 
> ((A + B) - C) % A   is equal to (B - C) % A
> 
> Which one is more readable ?

I'm fine with your suggestion.

#define BUFFER_ALIGN(adr) ((2 - ((u32) adr)) % XTE_ALIGN)

> 
> Please take a look at existing and clean code, no magic macro, and we
> can understand the intention.
> 
> find drivers/net | xargs grep -n netdev_alloc_skb_ip_align
> 
> 

Yes I see how it's used, but it only allows you to reserve 2 bytes in the skb with no options.




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