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

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Date:	Mon, 20 Jun 2011 17:04:23 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Nicolas Pitre <nico@...xnic.net>
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 Mon, 20 Jun 2011, Nicolas Pitre wrote:

> On Mon, 20 Jun 2011, Alan Stern wrote:
> 
> > On Mon, 20 Jun 2011, Alexander Holler wrote:
> > 
> > > I see it that way: packed is needed to be sure that at least for struct 
> > > ehci_regs there are no padding bytes inbetween the members.
> > 
> > But is it _really_ needed?
> > 
> > > It might 
> > > work without, but that depends on the compiler (-version, architecture, 
> > > whatever).
> > 
> > Have there _ever_ been _any_ combinations of compiler, version, 
> > architecture, whatever, that had unwanted padding bytes in this 
> > structure?
> 
> This can be determined by simple code inspection.
> 
> If you must have struct members which are not aligned to their natural 
> size then you need __packed.  Example:
> 
> struct foo {
> 	u8  a;
> 	u16 b;
> 	u32 c;
> 	u64 d;
> };
> 
> Without __packed, there will be padding between a and b, and between c 
> and d.

One byte of padding between a and b is enough.  No more is needed, and 
the compiler would have to be pretty stupid to add anything else.

>  If the order of the members in this struct were reversed, then 
> everything would be naturally aligned and no padding between members 
> would be inserted.
> 
> The size of structures is normally rounded up with padding to the size 
> of the largest basic element it contains.  Example:
> 
> struct foo {
> 	u64 a;
> 	u8 b;
> };
> 
> Here sizeof(struct foo) would return 16, even if the actual content 
> occupies 9 bytes only.  That's because the largest basic element is u64 
> i.e. 8 bytes.  Normally this trailing padding is not an issue, unless 
> you have an array of such a struct or if it is a member of another 
> struct.  If you want to get rid of that padding, you need to use 
> __packed again (which of course would make all subsequent instances of 
> that structure in your array completely misaligned too).
> 
> Two odd exceptions with the old ABI on ARM:
> 
> - The alignment of a 64-bit value is always 4 bytes not 8.
> 
> - The size of all structures are always rounded up to a 4-byte boundary, 
>   irrespective of their content.
> 
> If you fall into none of the above issues, then you don't need any 
> __packed, period.

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?

Alan Stern

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