Messages in this thread Patch in this message | | | Subject | [patch 2.6.13] document alignment and byteorder macros | From | Ed L Cashin <> | Date | Mon, 19 Sep 2005 15:22:01 -0400 |
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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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