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

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Message-ID: <4FC25048.109@wwwdotorg.org>
Date:	Sun, 27 May 2012 10:03:20 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	Jonathan Cameron <jic23@...nel.org>,
	Russell King <rmk@....linux.org.uk>
CC:	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 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.

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.
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