linux-kernel - Re: [PATCH] fix ehci alignment error

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Date:	Mon, 10 Oct 2011 19:41:55 +0100
From:	Måns Rullgård <mans@...sr.com>
To:	Greg KH <gregkh@...e.de>
Cc:	Mikael Pettersson <mikpe@...uu.se>,
	Måns Rullgård <mans@...sr.com>,
	Nicolas Pitre <nico@...xnic.net>, linux-usb@...r.kernel.org,
	Harro Haan <hrhaan@...il.com>, linux-kernel@...r.kernel.org,
	Alan Stern <stern@...land.harvard.edu>,
	Lennert Buytenhek <kernel@...tstofly.org>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] fix ehci alignment error

Greg KH <gregkh@...e.de> writes:

> On Mon, Oct 10, 2011 at 07:31:26PM +0200, Mikael Pettersson wrote:
>> Greg KH writes:
>>  > On Mon, Oct 10, 2011 at 05:29:13PM +0100, Måns Rullgård wrote:
>>  > > Greg KH <gregkh@...e.de> writes:
>>  > > 
>>  > > > On Mon, Oct 10, 2011 at 02:38:27PM +0200, Harro Haan wrote:
>>  > > >> The Kirkwood gave an unaligned memory access error on
>>  > > >> line 742 of drivers/usb/host/echi-hcd.c:
>>  > > >> "ehci->last_periodic_enable = ktime_get_real();"
>>  > > >
>>  > > > What is "The Kirkwood"?  What size processor is this?
>>  > > 
>>  > > It's an ARM926-like CPU from Marvell.
>>  > 
>>  > 32 or 64bit?
>> 
>> All ARMs are 32-bit, but some have instructions for loading or
>> storing 2 x 32 bits at a time, but also require the corresponding
>> memory address to be 64-bit aligned.

To be perfectly accurate, all ARMs have the dual load/store instructions,
but only some (ARMv5 and earlier) require a 64-bit aligned address for
these.  The Kirkwood is an ARMv5 so it does need this alignment.

Furthermore, the standard ARM ABI requires 8-byte alignment for 64-bit
data types regardless of CPU version, which is perhaps a bit unusual for
a 32-bit architecture.

-- 
Måns Rullgård
mans@...sr.com
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