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Subject: Re: share COMPATIBLE_IOCTL()s across architectures
From: Andi Kleen <ak@suse.de>
To: Pavel Machek <pavel@suse.cz>
Cc: "David S. Miller" <davem@redhat.com>, linux-kernel@vger.kernel.org,
       torvalds@transmeta.com
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Date: 20 Mar 2003 21:24:49 +0100
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On Thu, 2003-03-20 at 20:32, Pavel Machek wrote:
> Hi!
> 
> >    This patche moves common COMPATIBLE_IOCTLs to
> >    include/linux/compat_ioctl.h, enabling pretty nice cleanups:
> > 
> > Please be careful.  For anything non-trivial there can be major
> > differences between compat layers.
> 
> I'm trying to be carefull. How common are ioctls that are
> COMPATIBLE_IOCTL(foo) on one arch, but not on another? So far I tried
> to decide, and mostly decided that one architecture was simply
> missing...

The only issue I'm aware of are structures with long long. IA64 and
x86-64 are special in that long long has a different alignmnet in 32bit
and 64bit (4 bytes in 32bit, 8 bytes in 64bit). All the other archs with
compat code have always 8 byte alignment. This means if sparc64 doesn't
do a conversion, but x86-64 does you cannot put it into the COMPAT_IOCTL
list. Make sure you only use the common set.

Fortunately long long is not that common and many uses of it are already
8 byte aligned, but not all are.

-Andi



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