netdev - Re: [PATCH linux-next] net: batman-adv: use "__packed __aligned(2)" for each structure instead of "_

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Date:	Mon, 20 Jan 2014 11:28:41 +0000
From:	James Hogan <james.hogan@...tec.com>
To:	Antonio Quartulli <antonio@...hcoding.com>
CC:	Chen Gang <gang.chen.5i5j@...il.com>,
	David Miller <davem@...emloft.net>,
	<mareklindner@...mailbox.ch>, <sw@...onwunderlich.de>,
	<b.a.t.m.a.n@...ts.open-mesh.org>, netdev <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	<linux-metag@...r.kernel.org>
Subject: Re: [PATCH linux-next] net: batman-adv: use "__packed __aligned(2)"
 for each structure instead of "__packed(2)" region

Hi Antonio,

On 19/01/14 09:30, Antonio Quartulli wrote:
> On 19/01/14 02:10, James Hogan wrote:
>> It appears that the following gcc patch adds support for #pragma pack:
>> http://gcc.gnu.org/ml/gcc-patches/2006-10/msg01115.html
>>
>> I gave it a quick spin on metag gcc (which is unfortunately stuck on an old 
>> version) and it seems to fix my simple test case so that #pragma pack(2) 
>> becomes equivalent to __packed __aligned(2) (for sizeof and __alignof__).
>>
> 
> Then I personally think that it is better to fix metag gcc instead of
> changing the kernel.

Indeed it makes sense to patch metag gcc to be safe in the presence of
unportable code like this.

> Actually there are many different spots where "#pragma pack" is used.
> batman-adv is just the only one having compile time checks for structure
> sizes.

Well the only vaguely interesting one I can find outside of drivers is
in fs/udf, and even that seems specific to CD-ROMs and DVDs.

If you care about portability then Chen's patch looks reasonable to me.

Cheers
James


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