netdev - RE: [PATCH 2/9] ocrdma: Driver for Emulex OneConnect RDMA adapter

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Message-ID: <3ae9829d-f8dd-4268-918a-94616eff0915@exht1.ad.emulex.com>
Date:	Thu, 22 Mar 2012 14:20:28 -0700
From:	<Parav.Pandit@...lex.Com>
To:	<Parav.Pandit@...lex.Com>, <jgunthorpe@...idianresearch.com>
CC:	<David.Laight@...LAB.COM>, <roland@...estorage.com>,
	<linux-rdma@...r.kernel.org>, <netdev@...r.kernel.org>
Subject: RE: [PATCH 2/9] ocrdma: Driver for Emulex OneConnect RDMA adapter

I got a question here lately.

aligned directive will ensure that it will fall on boundary.
Say aligned(4) ensures that structure is aligned to 4 byte boundary.
Compiler can (at least theoretically) still have 4 byte structure aligned to 8 byte boundary on 64-bit platform (which is 4 byte aligned too).

struct {
	u32 field;
} attribute ((aligned(4));

However requirement is to have this structure only 4 byte size( because adapter excepts it to be 4B sise) and therefor packed is used.
I don't know the way to ensure size of 4 byte and alignment too.
Or I am misunderstanding?

Parav

> -----Original Message-----
> From: linux-rdma-owner@...r.kernel.org [mailto:linux-rdma-
> owner@...r.kernel.org] On Behalf Of Parav.Pandit@...lex.Com
> Sent: Friday, March 23, 2012 2:41 AM
> To: jgunthorpe@...idianresearch.com
> Cc: David.Laight@...LAB.COM; roland@...estorage.com; linux-
> rdma@...r.kernel.org; netdev@...r.kernel.org
> Subject: RE: [PATCH 2/9] ocrdma: Driver for Emulex OneConnect RDMA
> adapter
> 
> 
> 
> > -----Original Message-----
> > From: Jason Gunthorpe [mailto:jgunthorpe@...idianresearch.com]
> > Sent: Friday, March 23, 2012 2:28 AM
> > To: Pandit, Parav
> > Cc: David.Laight@...LAB.COM; roland@...estorage.com; linux-
> > rdma@...r.kernel.org; netdev@...r.kernel.org
> > Subject: Re: [PATCH 2/9] ocrdma: Driver for Emulex OneConnect RDMA
> > adapter
> >
> > On Thu, Mar 22, 2012 at 01:52:30PM -0700, Parav.Pandit@...lex.Com
> > wrote:
> >
> > > > This can be used to force 32bit alignment in amd64 code in order
> > > > to match definitions in 32bit userspace.
> > > > For new things it would make sense to force 64bit alignment of
> > > > 64bit fields for 32bit code.
> > >
> > > o.k. so I'll use aligned attribute to align user-kernel interface
> > > data structure to 8 byte boundary.  That should work for 32-bit and
> > > 64-bit user and kernel space and does't hurt performance either?
> >
> > If the structure is only for user/kernel interfacing then it is much
> > better to add explicit padding fields to naturally place 64 bit
> > quantities on an 8 byte alignment than to mess with gcc specific
> > attributes (user space has a much wide choice of compilers).
> >
> > This was David's second suggestion. Better to do this now before the
> > driver is accepted :)
> >
> o.k. I'll align them to naturally 8 byte boundary.
> 
> > Jason
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