linux-kernel - Re: [PATCH] USB: ehci: use packed,aligned(4) instead of removing the packed attribute

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Date:	Tue, 21 Jun 2011 16:41:59 -0400 (EDT)
From:	Nicolas Pitre <nico@...xnic.net>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Alexander Holler <holler@...oftware.de>,
	Arnd Bergmann <arnd@...db.de>,
	linux-arm-kernel@...ts.infradead.org, linux-usb@...r.kernel.org,
	gregkh@...e.de, lkml <linux-kernel@...r.kernel.org>,
	Rabin Vincent <rabin@....in>
Subject: Re: [PATCH] USB: ehci: use packed,aligned(4) instead of removing the
 packed attribute

On Tue, 21 Jun 2011, Alan Stern wrote:

> On Mon, 20 Jun 2011, Nicolas Pitre wrote:
> 
> > > We don't fall into any of these cases, and therefore as you say, we
> > > don't need packed.  Arnd and I have both explained this.  So why do you 
> > > keep arguing that we do need it?
> > 
> > Please show me where I keep arguing that you need it?
> 
> Not explicitly perhaps.  But you did write:
> 
> > Doesn't mean that because it used to work that it is strictly correct.  
> > Wouldn't be the first time that a GCC upgrade broke the kernel because 
> > the kernel wasn't describing things properly enough.
> 
> which strongly implies that "packed" is needed.  You also wrote:

In this case ...

> > Yes, but that's a consequence of not being able to access those fields 
> > in their naturally aligned position anymore.  Hence the addition of the 
> > align attribute to tell the compiler that we know that the structure is 
> > still aligned to a certain degree letting the compiler to avoid 
> > byte-oriented instructions when possible.
> 
> which is predicated on the assumption that "packed" is needed.

... and also in this case, I was talking about proper use of the packed 
attribute in general, not at all about a specific case.  I wanted to 
provide a broader view to some people who expressed doubts and 
misunderstanding in the hope that the archive could keep this knowledge 
base available.

I apologize if that wasn't clear to you.


Nicolas
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