Message-ID: <86f149a4-088a-4c37-ba6f-a49dbc9acbeb@VA3EHSMHS033.ehs.local>
Date:	Tue, 6 Apr 2010 14:03:38 -0600
From:	John Linn <John.Linn@...inx.com>
To:	"Grant Likely" <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: glikely@...retlab.ca [mailto:glikely@...retlab.ca] On Behalf Of Grant Likely
> Sent: Tuesday, April 06, 2010 12:54 PM
> To: John Linn
> Cc: 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, Apr 6, 2010 at 11:11 AM, John Linn <John.Linn@...inx.com> wrote:
> >> -----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.
> 
> 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.
> 
> >> > 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.
> 
> Eric is here.  The mod operation means that BUFFER_ALIGN using either
> 2 or 34 is equivalent.
> 
> g.

I can spin another patch with the following and with Grant's Kconfig changes, just looking for confirmation that's acceptable.

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

Thanks,
John




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