linux-kernel - Re: [PATCH] include/linux/unaligned: Pack the whole struct rather than just the field.

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Date:	Sat, 18 Dec 2010 12:59:23 +0000
From:	Will Newton <will.newton@...il.com>
To:	Linux Kernel list <linux-kernel@...r.kernel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH] include/linux/unaligned: Pack the whole struct rather
 than just the field.

On Tue, Dec 7, 2010 at 11:12 AM, Will Newton <will.newton@...il.com> wrote:
> On Wed, Dec 1, 2010 at 10:11 PM, Will Newton <will.newton@...il.com> wrote:
>> The current packed struct implementation of unaligned access adds
>> the packed attribute only to the field within the unaligned struct
>> rather than to the struct as a whole. This is not sufficient to
>> enforce proper behaviour on architectures with a default struct
>> alignment of more than one byte.
>>
>> For example, the current implementation of __get_unaligned_cpu16
>> when compiled for arm with gcc -O1 -mstructure-size-boundary=32
>> assumes the struct is on a 4 byte boundary so performs the load
>> of the 16bit packed field as if it were on a 4 byte boundary:
>>
>> __get_unaligned_cpu16:
>>        ldrh    r0, [r0, #0]
>>        bx      lr
>>
>> Moving the packed attribute to the struct rather than the field
>> causes the proper unaligned access code to be generated:
>>
>> __get_unaligned_cpu16:
>>        ldrb    r3, [r0, #0]    @ zero_extendqisi2
>>        ldrb    r0, [r0, #1]    @ zero_extendqisi2
>>        orr     r0, r3, r0, asl #8
>>        bx      lr
>>
>> Signed-off-by: Will Newton <will.newton@...il.com>
>
> I don't know of any designated maintainer for this code, but add a
> couple of ccs of people who might be interested.
>
> This change doesn't change any behaviour on current architectures, but
> is more correct and may enable other architectures to move to the
> packed struct unaligned access implementation and so allow us to move
> more architectures to the asm-generic implementation.

Ping!

Does anyone have any opinions on this?

>> ---
>>  include/linux/unaligned/packed_struct.h |    6 +++---
>>  1 files changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/include/linux/unaligned/packed_struct.h
>> b/include/linux/unaligned/packed_struct.h
>> index 2498bb9..c9a6abd 100644
>> --- a/include/linux/unaligned/packed_struct.h
>> +++ b/include/linux/unaligned/packed_struct.h
>> @@ -3,9 +3,9 @@
>>
>>  #include <linux/kernel.h>
>>
>> -struct __una_u16 { u16 x __attribute__((packed)); };
>> -struct __una_u32 { u32 x __attribute__((packed)); };
>> -struct __una_u64 { u64 x __attribute__((packed)); };
>> +struct __una_u16 { u16 x; } __attribute__((packed));
>> +struct __una_u32 { u32 x; } __attribute__((packed));
>> +struct __una_u64 { u64 x; } __attribute__((packed));
>>
>>  static inline u16 __get_unaligned_cpu16(const void *p)
>>  {
>> --
>> 1.7.0.4
>>
>
--
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