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introduce HAVE_EFFICIENT_UNALIGNED_ACCESS Kconfig symbol

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In many cases, especially in networking, it can be beneficial to know at
compile time whether the architecture can do unaligned accesses efficiently.
This patch introduces a new Kconfig symbol

	HAVE_EFFICIENT_UNALIGNED_ACCESS

for that purpose and adds it to the powerpc and x86 architectures.  Also add
some documentation about alignment and networking, and especially one intended
use of this symbol.

Signed-off-by: Johannes Berg <[email protected]>
Acked-by: Sam Ravnborg <[email protected]>
Acked-by: Ingo Molnar <[email protected]> [x86 architecture part]
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
This commit is contained in:
Johannes Berg 2008-07-25 01:45:33 -07:00 • committed by Linus Torvalds
commit 58340a07c1
4 changed files with 50 additions and 3 deletions
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19
arch/Kconfig
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@ -27,6 +27,25 @@ config KPROBES
for kernel debugging, non-intrusive instrumentation and testing.
If in doubt, say "N".
config HAVE_EFFICIENT_UNALIGNED_ACCESS
def_bool n
help
Some architectures are unable to perform unaligned accesses
without the use of get_unaligned/put_unaligned. Others are
unable to perform such accesses efficiently (e.g. trap on
unaligned access and require fixing it up in the exception
handler.)
This symbol should be selected by an architecture if it can
perform unaligned accesses efficiently to allow different
code paths to be selected for these cases. Some network
drivers, for example, could opt to not fix up alignment
problems with received packets if doing so would not help
much.
See Documentation/unaligned-memory-access.txt for more
information on the topic of unaligned memory accesses.
config KRETPROBES
def_bool y
depends on KPROBES && HAVE_KRETPROBES