netdev - Re: [PATCH v4 05/10] net/fec: add dual fec support for mx28

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Date:	Mon, 17 Jan 2011 19:52:05 +0800
From:	Shawn Guo <shawn.guo@...escale.com>
To:	Lothar Waßmann <LW@...O-electronics.de>
CC:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>, <gerg@...pgear.com>,
	<B32542@...escale.com>, <netdev@...r.kernel.org>,
	<s.hauer@...gutronix.de>, <jamie@...reable.org>,
	<baruch@...s.co.il>, <w.sang@...gutronix.de>,
	<r64343@...escale.com>, <eric@...rea.com>,
	<bryan.wu@...onical.com>, <jamie@...ieiles.com>,
	<davem@...emloft.net>, <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v4 05/10] net/fec: add dual fec support for mx28

Hi Lothar,

On Mon, Jan 17, 2011 at 09:16:22AM +0100, Lothar Waßmann wrote:
> Hi,
> 
> Shawn Guo writes:
> > On Fri, Jan 14, 2011 at 08:52:23AM +0100, Uwe Kleine-König wrote:
> > > On Fri, Jan 14, 2011 at 01:48:40PM +0800, Shawn Guo wrote:
> > > > Hi Uwe,
> > > > 
> > > > On Thu, Jan 13, 2011 at 03:48:05PM +0100, Uwe Kleine-König wrote:
> > > > 
> > > > [...]
> > > > 
> > > > > > +/* Controller is ENET-MAC */
> > > > > > +#define FEC_QUIRK_ENET_MAC           (1 << 0)
> > > > > does this really qualify to be a quirk?
> > > > > 
> > > > My understanding is that ENET-MAC is a type of "quirky" FEC
> > > > controller.
> > > > 
> > > > > > +/* Controller needs driver to swap frame */
> > > > > > +#define FEC_QUIRK_SWAP_FRAME         (1 << 1)
> > > > > IMHO this is a bit misnamed.  FEC_QUIRK_NEEDS_BE_DATA or similar would
> > > > > be more accurate.
> > > > > 
> > > > When your make this change, you may want to pick a better name for
> > > > function swap_buffer too.
> > > > 
> > > > [...]
> > > > 
> > > > > > +static void *swap_buffer(void *bufaddr, int len)
> > > > > > +{
> > > > > > +     int i;
> > > > > > +     unsigned int *buf = bufaddr;
> > > > > > +
> > > > > > +     for (i = 0; i < (len + 3) / 4; i++, buf++)
> > > > > > +             *buf = cpu_to_be32(*buf);
> > > > > if len isn't a multiple of 4 this accesses bytes behind len.  Is this
> > > > > generally OK here?  (E.g. because skbs always have a length that is a
> > > > > multiple of 4?)
> > > > The len may not be a multiple of 4.  But I believe bufaddr is always
> > > > a buffer allocated in a length that is a multiple of 4, and the 1~3
> > > > bytes exceeding the len very likely has no data that matters.  But
> > > > yes, it deserves a safer implementation.
> > > Did you test what happens if bufaddr isn't aligned?  Does it work at all
> > > then?
> > > 
> > I see many calls passing a len that is not a multiple of 4, but it
> > works good.
> > 
> That does not prove anything, actually.
> 
> Anyway "bufaddr isn't aligned" != "len is not a multiple of 4".
> Is there any guarantee that the function cannot be called with a
> non-aligned buffer address?
> 
Oops, I misunderstood the comment.  With bounce buffer alignment
handling removed, the driver stops working.  But at least, mx28
fec driver can work with FEC_ALIGNMENT 0x3 and not necessarily with
0xf. 

I hope this is what you intended to know.

-- 
Regards,
Shawn

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