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-u79uwXL29TY76Z2rM5mHXA@public.gmane.org [mailto:linux-rdma-
owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org] On Behalf Of Parav.Pandit-iH1Dq9VlAzfQT0dZR+AlfA@public.gmane.org
Sent: Friday, March 23, 2012 2:41 AM
To: jgunthorpe-ePGOBjL8dl3ta4EC/59zMFaTQe2KTcn/@public.gmane.org
Cc: David.Laight-ZS65k/vG3HxXrIkS9f7CXA@public.gmane.org; roland-BHEL68pLQRGGvPXPguhicg@public.gmane.org; linux-
rdma-u79uwXL29TY76Z2rM5mHXA@public.gmane.org; netdev-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
Subject: RE: [PATCH 2/9] ocrdma: Driver for Emulex OneConnect RDMA
adapter

quoted

-----Original Message-----
From: Jason Gunthorpe [mailto:jgunthorpe-ePGOBjL8dl3ta4EC/59zMFaTQe2KTcn/@public.gmane.org]
Sent: Friday, March 23, 2012 2:28 AM
To: Pandit, Parav
Cc: David.Laight-ZS65k/vG3HxXrIkS9f7CXA@public.gmane.org; roland-BHEL68pLQRGGvPXPguhicg@public.gmane.org; linux-
rdma-u79uwXL29TY76Z2rM5mHXA@public.gmane.org; netdev-u79uwXL29TY76Z2rM5mHXA@public.gmane.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-iH1Dq9VlAzfQT0dZR+AlfA@public.gmane.org
wrote:

quoted

quoted

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.

quoted

Jason

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