linux-kernel - Re: [RFC v2] Documentation about unaligned memory access

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Message-ID: <5eeb9ad90711292358q2e8cf5few8d9c1b99195e1ee@mail.gmail.com>
Date:	Fri, 30 Nov 2007 08:58:28 +0100
From:	DM <dm.n9107@...il.com>
To:	"Daniel Drake" <dsd@...too.org>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [RFC v2] Documentation about unaligned memory access

On Nov 29, 2007 5:15 PM, Daniel Drake <dsd@...too.org> wrote:
[...]
> To avoid the unaligned memory access, you would rewrite it as follows:
>
>        void myfunc(u8 *data, u32 value)
>        {
>                [...]
>                value = cpu_to_le32(value);
>                put_unaligned(value, data);
>                [...]
>        }
>
> The get_unaligned() macro works similarly. Assuming 'data' is a pointer to
> memory and you wish to avoid unaligned access, its usage is as follows:
>
>        u32 value = get_unaligned(data);
>
> These macros work work for memory accesses of any length (not just 32 bits as
> in the examples above). Be aware that when compared to standard access of
> aligned memory, using these macros to access unaligned memory can be costy in
> terms of performance.
>

The get_unaligned call above will not do what you intended given the,
at least as I read it, implied context of myfunc. Since data is a u8*
it will only get one byte of data. To avoid misunderstandings the code
should probably read:

u32 value = get_unaligned((u32 *)data);

/DM
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