netdev - RE: [PATCH net] net/mlx4_en: correct the endianness of doorbell

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Date:	Mon, 15 Dec 2014 10:00:18 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Wei Yang' <weiyang@...ux.vnet.ibm.com>,
	David Miller <davem@...emloft.net>
CC:	"eric.dumazet@...il.com" <eric.dumazet@...il.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"gideonn@...lanox.com" <gideonn@...lanox.com>,
	"edumazet@...gle.com" <edumazet@...gle.com>,
	"amirv@...lanox.com" <amirv@...lanox.com>
Subject: RE: [PATCH net] net/mlx4_en: correct the endianness of doorbell_qpn
 on big endian platform

From: Wei Yang
> On Sat, Dec 13, 2014 at 11:43:20PM -0500, David Miller wrote:
> >From: Wei Yang <weiyang@...ux.vnet.ibm.com>
> >Date: Sat, 13 Dec 2014 11:13:38 +0800
> >
> >> On Mon, Dec 08, 2014 at 10:42:37PM +0800, Wei Yang wrote:
> >> If you prefer this way, I would like to send a new version for this.
> >> Is it ok for you?
> >
> >I'm not so sure.  There are implications when using the __raw_*()
> >routines.
> >
> >In particular, using __raw_{read,write}l() also means that the usual
> >necessary I/O memory barriers are not being performed.
> >
> >There are therefore no ordering guarantees between __raw_*() and other
> >I/O or memory accesses whatsoever.
> 
> Thanks David.
> 
> Actually, the last mail is asking David Laight. I am trying to understanding
> his comment and Amir told me he was suggesting to use __raw_*() version.

I don't know (off hand) which of the access functions contain byteswaps and
memory barriers, from trying to get some drivers running on ppc I know the
whole thing is a complete mess (beats me why all the on-chip devices of
opposite endianness to the cpu iteself).

In this case you seem to have an earlier change to store the value that
doesn't need a byteswap on write (for efficiency), then are using an
access function that does a byteswapping write to memory (efficient, but
the wrong value), and are fixing it by adding another byteswap (inefficient)
prior to the byteswapping write.

So you should either be using a non-byteswapping write, of just save
the byteswapped value so that the written value is correct.

	David

> Hmm... this is really a problem found in the v3.18-rc1 and the root cause is
> the endianess. I am ok to use any method to fix this problem, even revert it.
> Could the maintainer from Mellanox gives me a word?
> 
> --
> Richard Yang
> Help you, Help me



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