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Subject[patch 2.6.13] document alignment and byteorder macros
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"David S. Miller" <davem@davemloft.net> writes:

> From: Ed L Cashin <ecashin@coraid.com>
> Date: Mon, 19 Sep 2005 10:24:00 -0400
>
>> 1) Passing le64_to_cpup an unaligned pointer is "OK" and within the
>> intended use of the function. I'm having trouble finding whether
>> this is documented somewhere.
>>
>> 2) These new changes to the sparc64 unaligned access fault handling
>> will make it OK to leave the aoe driver the way it is in the
>> mainline kernel.
>
> Both #1 and #2 are true.
>
> Although it's very much discouraged to dereference unaligned pointers,
> especially in performance critical code (which this AOE case is not,
> thankfully), because performance will be really bad as the trap
> handler has to fix up the access on RISC platforms.

Roland Dreier <rolandd@cisco.com> writes:

> David> Although it's very much discouraged to dereference
> David> unaligned pointers, especially in performance critical code
> David> (which this AOE case is not, thankfully), because
> David> performance will be really bad as the trap handler has to
> David> fix up the access on RISC platforms.
>
> Also, ia64 has a tendency to print an ugly message in the kernel log
> for unaligned accesses. Has anyone tried AoE on ia64?
>
> It might be better to change the AoE code to use get_unaligned(), just
> to document what's going on. Although clearly the sparc64 patch is
> correct as well -- we should never silently return the wrong data.

This patch comments the fact that although passing le64_to_cpup et
al. is within the intended use of the byteorder macros, using
get_unaligned is the recommended way to go.

Please speak up if there's a better place for this documentation to go
or a better way to say it.


document alignment and byteorder macros

Signed-off-by: Ed L. Cashin <ecashin@coraid.com>

Index: linux-2.6.13/include/linux/byteorder/generic.h
===================================================================
--- linux-2.6.13.orig/include/linux/byteorder/generic.h 2005-08-31 17:00:15.000000000 -0400
+++ linux-2.6.13/include/linux/byteorder/generic.h 2005-09-19 15:15:37.000000000 -0400
@@ -5,6 +5,10 @@
* linux/byteorder_generic.h
* Generic Byte-reordering support
*
+ * The "... p" macros, like le64_to_cpup, can be used with pointers
+ * to unaligned data, but there will be a performance penalty on
+ * some architectures. Use get_unaligned for unaligned data.
+ *
* Francois-Rene Rideau <fare@tunes.org> 19970707
* gathered all the good ideas from all asm-foo/byteorder.h into one file,
* cleaned them up.

--
Ed L Cashin <ecashin@coraid.com>
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