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Message-ID: <Pine.LNX.4.44L0.1106211058250.2327-100000@iolanthe.rowland.org>
Date: Tue, 21 Jun 2011 11:06:37 -0400 (EDT)
From: Alan Stern <stern@...land.harvard.edu>
To: Nicolas Pitre <nico@...xnic.net>
cc: Arnd Bergmann <arnd@...db.de>,
<linux-arm-kernel@...ts.infradead.org>,
Alexander Holler <holler@...oftware.de>,
<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, Nicolas Pitre wrote:
> >
> > > > As far as I can tell, the other structures in ehci.h have
> > > > ((aligned(32)) simply in order to save space, since there can be large
> > > > numbers of these structures allocated.
> > >
> > > How can increasing the alignment to 32 bytes save space?
> >
> > No, no -- the alignment is _decreased_ to 32 bits. Without the
> > attribute the alignment would have been 64 bits.
>
> The aligned attribute requires a byte value not a bit value.
> Maybe what you meant is ((aligned(4)) ?
Ah, very good point! No, I meant ((aligned(32))). Do "grep aligned
drivers/usb/host/ehci.h" and you'll see.
So my understanding was wrong; these structure really are being forced
to a strict alignment. And indeed, now that I go back and look at the
EHCI spec, it turns out that this alignment is required by the
hardware.
Okay, so this digression was irrelevant to our discussion. Forget I
mentioned it...
Alan Stern
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