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Subject[PATCH/RFC v4] introduce HAVE_UNALIGNED_ACCESS_SUPPORT 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. This patch introduces a new Kconfig symbol
HAVE_UNALIGNED_ACCESS_SUPPORT
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 <johannes@sipsolutions.net>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
---
Didn't I say I was going to fix the subject? Sorry.
Documentation/unaligned-memory-access.txt | 32 +++++++++++++++++++++++++++---
arch/Kconfig | 3 ++
arch/powerpc/Kconfig | 1
arch/x86/Kconfig | 1
4 files changed, 34 insertions(+), 3 deletions(-)
--- everything.orig/Documentation/unaligned-memory-access.txt 2008-03-20 15:30:38.000000000 +0100
+++ everything/Documentation/unaligned-memory-access.txt 2008-03-20 19:38:30.000000000 +0100
@@ -218,9 +218,35 @@ If use of such macros is not convenient,
where the source or destination (or both) are of type u8* or unsigned char*.
Due to the byte-wise nature of this operation, unaligned accesses are avoided.

+
+Alignment vs. Networking
+========================
+
+On architectures that require aligned loads, networking requires that the IP
+header is aligned on a four-byte boundary to optimise the IP stack. For
+regular ethernet hardware, the constant NET_IP_ALIGN is used, on most
+architectures this constant has the value 2 because the normal ethernet
+header is 14 bytes long, so in order to get proper alignment one needs to
+DMA to an address that is can be expressed as 4*n + 2. One notable exception
+here is powerpc which defines NET_IP_ALIGN to 0 because DMA to unaligned
+addresses can be very expensive and dwarf the cost of unaligned loads.
+
+For some ethernet hardware that cannot DMA to unaligned addresses like
+4*n+2 or non-ethernet hardware, this can be a problem, and it is then
+required to copy the incoming frame into an aligned buffer. Because this is
+unnecessary on architectures that can do unaligned accesses, the code can be
+made depend on CONFIG_HAVE_UNALIGNED_ACCESS_SUPPORT like so:
+
+#ifdef CONFIG_HAVE_UNALIGNED_ACCESS_SUPPORT
+ skb = original skb
+#else
+ skb = copy skb
+#endif
+
--
-Author: Daniel Drake <dsd@gentoo.org>
+Authors: Daniel Drake <dsd@gentoo.org>,
+ Johannes Berg <johannes@sipsolutions.net>
With help from: Alan Cox, Avuton Olrich, Heikki Orsila, Jan Engelhardt,
-Johannes Berg, Kyle McMartin, Kyle Moffett, Randy Dunlap, Robert Hancock,
-Uli Kunitz, Vadim Lobanov
+Kyle McMartin, Kyle Moffett, Randy Dunlap, Robert Hancock, Uli Kunitz,
+Vadim Lobanov

--- everything.orig/arch/powerpc/Kconfig 2008-03-20 15:30:38.000000000 +0100
+++ everything/arch/powerpc/Kconfig 2008-03-20 19:37:22.000000000 +0100
@@ -91,6 +91,7 @@ config PPC
select HAVE_OPROFILE
select HAVE_KPROBES
select HAVE_KRETPROBES
+ select HAVE_UNALIGNED_ACCESS_SUPPORT

config EARLY_PRINTK
bool
--- everything.orig/arch/x86/Kconfig 2008-03-20 15:30:38.000000000 +0100
+++ everything/arch/x86/Kconfig 2008-03-20 19:38:08.000000000 +0100
@@ -23,6 +23,7 @@ config X86
select HAVE_KPROBES
select HAVE_KRETPROBES
select HAVE_KVM if ((X86_32 && !X86_VOYAGER && !X86_VISWS && !X86_NUMAQ) || X86_64)
+ select HAVE_UNALIGNED_ACCESS_SUPPORT


config GENERIC_LOCKBREAK
--- everything.orig/arch/Kconfig 2008-03-20 19:37:26.000000000 +0100
+++ everything/arch/Kconfig 2008-03-20 19:37:34.000000000 +0100
@@ -36,3 +36,6 @@ config HAVE_KPROBES

config HAVE_KRETPROBES
def_bool n
+
+config HAVE_UNALIGNED_ACCESS_SUPPORT
+ def_bool n



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