The current implementation of from64to32() gives a poor result:
0000000000000270 <.from64to32>:
270: 38 00 ff ff li r0,-1
274: 78 69 00 22 rldicl r9,r3,32,32
278: 78 00 00 20 clrldi r0,r0,32
27c: 7c 60 00 38 and r0,r3,r0
280: 7c 09 02 14 add r0,r9,r0
284: 78 09 00 22 rldicl r9,r0,32,32
288: 7c 00 4a 14 add r0,r0,r9
28c: 78 03 00 20 clrldi r3,r0,32
290: 4e 80 00 20 blr
This patch modifies from64to32() to operate in the same
spirit as csum_fold()
It swaps the two 32-bit halves of sum then it adds it with the
unswapped sum. If there is a carry from adding the two 32-bit halves,
it will carry from the lower half into the upper half, giving us the
correct sum in the upper half.
The resulting code is:
0000000000000260 <.from64to32>:
260: 78 60 00 02 rotldi r0,r3,32
264: 7c 60 1a 14 add r3,r0,r3
268: 78 63 00 22 rldicl r3,r3,32,32
26c: 4e 80 00 20 blr
Signed-off-by: Christophe Leroy <christophe.le...@c-s.fr>
---
arch/powerpc/include/asm/checksum.h | 7 ++-----
1 file changed, 2 insertions(+), 5 deletions(-)
diff --git a/arch/powerpc/include/asm/checksum.h
b/arch/powerpc/include/asm/checksum.h
index 4e63787dc3be..54065caa40b3 100644
--- a/arch/powerpc/include/asm/checksum.h
+++ b/arch/powerpc/include/asm/checksum.h
@@ -12,6 +12,7 @@
#ifdef CONFIG_GENERIC_CSUM
#include <asm-generic/checksum.h>
#else
+#include <linux/bitops.h>
/*
* Computes the checksum of a memory block at src, length len,
* and adds in "sum" (32-bit), while copying the block to dst.
@@ -55,11 +56,7 @@ static inline __sum16 csum_fold(__wsum sum)
static inline u32 from64to32(u64 x)
{
- /* add up 32-bit and 32-bit for 32+c bit */
- x = (x & 0xffffffff) + (x >> 32);
- /* add up carry.. */
- x = (x & 0xffffffff) + (x >> 32);
- return (u32)x;
+ return (x + ror64(x, 32)) >> 32;
}
static inline __wsum csum_tcpudp_nofold(__be32 saddr, __be32 daddr, __u32 len,
--
2.13.3
