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

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Message-ID: <20110117084221.GM6917@pengutronix.de>
Date:	Mon, 17 Jan 2011 09:42:21 +0100
From:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
To:	Lothar Waßmann <LW@...O-electronics.de>
Cc:	Shawn Guo <shawn.guo@...escale.com>, 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,

On Mon, Jan 17, 2011 at 09:16:22AM +0100, Lothar Waßmann wrote:
> 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?
Over the weekend I wondered if we could reach alignment via a dma-mask
setting.  Didn't check yet how this is configured.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |
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