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

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Message-ID: <CAPXgP10gLk1UPp8iGkDSapwMzqd2kHuOer3BOq73f5i7if4aoA@mail.gmail.com>
Date:	Tue, 29 May 2012 18:32:21 +0200
From:	Kay Sievers <kay@...y.org>
To:	Stephen Warren <swarren@...dotorg.org>
Cc:	Jonathan Cameron <jic23@...nel.org>,
	Russell King <rmk@....linux.org.uk>,
	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 Tue, May 29, 2012 at 6:14 PM, Stephen Warren <swarren@...dotorg.org> wrote:
> On 05/27/2012 10:03 AM, 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.
> ...
>> #if !defined(CONFIG_64BIT) || defined(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)
>> #define LOG_ALIGN 4
>> #else
>> #define LOG_ALIGN 8
>> #endif
>
> Actually, why not replace that with:
>
> #define LOG_ALIGN (__alignof__(struct log_buf))
>
> That way, the compiler will calculate the arch-/ABI-appropriate
> alignment value automatically and correctly in all cases, so we won't
> have to fix that ifdef above.

__alignof__(u64) will be 8 on x86_64, while the current logic results
in 4. Not sure if x86_64 would somehow benefit from that, or if it's
just a waste of bytes.

Are you sure it results in 4 on some architectures?

Kay
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