linux-kernel - Re: [PATCH 2/3] Import fw-ohci driver.

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Message-ID: <457C913A.20600@s5r6.in-berlin.de>
Date:	Sun, 10 Dec 2006 23:59:06 +0100
From:	Stefan Richter <stefanr@...6.in-berlin.de>
To:	Kristian Høgsberg <krh@...hat.com>
CC:	Jeff Garzik <jeff@...zik.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] Import fw-ohci driver.

Kristian Høgsberg wrote:
> Stefan Richter wrote:
>> Kristian Høgsberg wrote:
>>> Yup, I've done away with the bitfields and switched to a mix of __le16
>>> and __le32 struct fields.
>>
>> I suppose the struct should get __attribute__((packed)) then.
> 
> I guess it wouldn't harm, but is it really necessary?  Would gcc ever
> insert padding here, all the 32 bit fields a 32 bit aligned, and so are
> the 16 bit fields.

Is 2-byte alignment of 16bit struct members guaranteed on all platforms?

A related question:
If I specify a struct which, among else, contains 32bit quantities, then
any variable of this struct type is supposed to be at least 4-byte-aligned.
No if I specifiy this struct as packed, will variables of this type still
be aligned on 4 byte boundaries or will the compiler assume no alignment?
In other words, should it be __attribute__((packed,aligned(4))) then?
I'm speaking about situations where I not only wish to avoid unnecessarily
bad machine code due to unaligned access but where the device requires
4-byte alignment too.
-- 
Stefan Richter
-=====-=-==- ==-- -=-=-
http://arcgraph.de/sr/
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