netdev - Re: [PATCH] drivers: staging: lustre: lustre: include: add "__attribute_

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Date:	Mon, 03 Feb 2014 18:25:41 +0800
From:	Chen Gang <gang.chen.5i5j@...il.com>
To:	David Laight <David.Laight@...LAB.COM>
CC:	'Dan Carpenter' <dan.carpenter@...cle.com>,
	James Hogan <james.hogan@...tec.com>,
	"devel@...verdev.osuosl.org" <devel@...verdev.osuosl.org>,
	"andreas.dilger@...el.com" <andreas.dilger@...el.com>,
	Antonio Quartulli <antonio@...hcoding.com>,
	Greg KH <gregkh@...uxfoundation.org>,
	"bergwolf@...il.com" <bergwolf@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	"oleg.drokin@...el.com" <oleg.drokin@...el.com>,
	"jacques-charles.lafoucriere@....fr" 
	<jacques-charles.lafoucriere@....fr>,
	"jinshan.xiong@...el.com" <jinshan.xiong@...el.com>,
	netdev <netdev@...r.kernel.org>,
	"linux-metag@...r.kernel.org" <linux-metag@...r.kernel.org>
Subject: Re: [PATCH] drivers: staging: lustre: lustre: include: add "__attribute__((packed))"
 for the related union

On 02/03/2014 06:05 PM, David Laight wrote:
> From: Dan Carpenter
>> On Sat, Feb 01, 2014 at 09:57:39PM +0800, Chen Gang wrote:
>>> It seems, our kernel still stick to treate 'pack' region have effect
>>> with both 'align' and 'sizeof'.
>>
>> It's not about packed regions.  It's about unions.  It's saying the
>> sizeof() a union is a multiple of 4 unless it's packed.
>>
>> union foo {
>> 	short x;
>> 	short y;
>> };
>>
>> The author intended the sizeof(union foo) to be 2 but on metag arch then
>> it is 4.
> 
> The same is probably be true of: struct foo { _u16 bar; };
> 

I guess so.


> Architectures that define such alignment rules are a right PITA.

Sorry, I do not know about PITA (after google or wiki, I can not get
more related information).

Could you provide more information about PITA, thanks?


> You either need to get the size to 2 without using 'packed', or
> just not define such structures.

Excuse me, I don't quite understand your meaning. I guess your meaning
is:

  "normally, we should not use a struct/union like that, no matter what it is (2 or 4)".

Is it correct.


> It is worth seeing if adding aligned(2) will change the size - I'm
> not sure.
> 

Yes, it will/should make sure that it must be 2.


Thanks.
-- 
Chen Gang

Open, share and attitude like air, water and life which God blessed
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