linux-kernel - Re: arm: Remaining issue with alignment of __log

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Message-ID: <20120527161405.GA29360@n2100.arm.linux.org.uk>
Date:	Sun, 27 May 2012 17:14:05 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Stephen Warren <swarren@...dotorg.org>
Cc:	Jonathan Cameron <jic23@...nel.org>, kay.sievers@...y.org,
	LKML <linux-kernel@...r.kernel.org>,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: arm: Remaining issue with alignment of __log_buf in printk.c

On Sun, May 27, 2012 at 10:03:20AM -0600, Stephen Warren wrote:
> On 05/27/2012 06:39 AM, Jonathan Cameron wrote:
> > Hi All,
> > 
> > My stargate 2 board refused to start and after bisection I ended
> > up at the same patch that Stephen found an alignment issue in.
> > Unfortunately Stephen's patch doesn't seem to have fixed the
> > issue for me.
> > https://lkml.org/lkml/2012/5/10/510 is the thread.  Patch from
> > Stephen is : f8450fca6ecdea38b5a882fdf6cd097e3ec8651c
> > 
> > Increasing the alignement for 32 bit systems to 8 seems to do the
> > job but I can't immediately think why...
> > 
> > System is a pxa27x strong arm.

First thing to point out is that PXA27x is Xscale, not StrongARM.

> The first element in the structure type that's actually stored in the
> __log_buf array is a u64; see struct log in kernel/printk.c. Depending
> on alignment rules, a u64 and a struct containing it might require a
> 4-byte or 8-byte alignment. The following link implies this might have
> changed over time:
> 
> http://wiki.debian.org/ArmEabiPort#Struct_packing_and_alignment
> 
> (see "64-bit data type alignment" a little below that anchor). I'm not
> sure what ABI the kernel expects to use internally, or your compiler;
> perhaps you need the new EABI 8-byte alignment requirement for a u64 and
> hence the struct as a whole, but Tegra (or my toolchain?) is OK with the
> older 4-byte alignment for a u64 or struct?
> 
> Further, I'm not sure if the following alignment selection logic:
> 
> > #if !defined(CONFIG_64BIT) || defined(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)
> > #define LOG_ALIGN 4
> > #else
> > #define LOG_ALIGN 8
> > #endif
> 
> ... uses the CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS for its intended
> purpose?
> 
> Russell, can you please comment here. Thanks.

And most likely it's using EABI which does want 8 byte alignment.  So this
should probably be fixed for EABI builds.
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