Date: Mon, 28 Jun 2004 14:03:43 -0700
From: "David S. Miller" <davem@redhat.com>
To: Oliver Neukum <oliver@neukum.org>
Cc: scott@timesys.com, zaitcev@redhat.com, greg@kroah.com, arjanv@redhat.com,
       jgarzik@redhat.com, tburke@redhat.com, linux-kernel@vger.kernel.org,
       stern@rowland.harvard.edu, mdharm-usb@one-eyed-alien.net,
       david-b@pacbell.net
Subject: Re: drivers/block/ub.c
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In-Reply-To: <200406282257.11026.oliver@neukum.org>
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	<200406282257.11026.oliver@neukum.org>
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On Mon, 28 Jun 2004 22:57:11 +0200
Oliver Neukum <oliver@neukum.org> wrote:

> Am Montag, 28. Juni 2004 22:25 schrieb David S. Miller:
> > That's true. ?But if one were to propose such a feature to the gcc
> > guys, I know the first question they would ask. ?"If no padding of
> > the structure is needed, why are you specifying this new
> > __nopadding__ attribute?"
> 
> It would replace some uses of __packed__, where the first element
> is aligned.

You have not considered what is supposed to happen when this
structure is embedded within another one.  What kind of alignment
rules apply in that case?  For example:

struct foo {
	u32	x;
	u8	y;
	u16	z;
} __attribute__((__packed__));

struct bar {
	u8		a;
	struct foo 	b;
};

That is why __packed__ can't assume the alignment of any structure
instance whatsoever.  Your __nopadding__ attribute proposal would
lay out struct bar differently in order to meet the alignment guarentees
you say it will be able to meet.
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