From 96f391cf40ee5c9201cc7b55abe3903761e6a2e2 Mon Sep 17 00:00:00 2001
From: Christophe Leroy <christophe.leroy@c-s.fr>
Date: Tue, 10 Apr 2018 08:34:37 +0200
Subject: [PATCH] Revert "powerpc/64: Fix checksum folding in csum_add()"
References: bsc#1065729
Patch-mainline: v4.18-rc1
Git-commit: 96f391cf40ee5c9201cc7b55abe3903761e6a2e2
This reverts commit 6ad966d7303b70165228dba1ee8da1a05c10eefe.
That commit was pointless, because csum_add() sums two 32 bits
values, so the sum is 0x1fffffffe at the maximum.
And then when adding upper part (1) and lower part (0xfffffffe),
the result is 0xffffffff which doesn't carry.
Any lower value will not carry either.
And behind the fact that this commit is useless, it also kills the
whole purpose of having an arch specific inline csum_add()
because the resulting code gets even worse than what is obtained
with the generic implementation of csum_add()
0000000000000240 <.csum_add>:
240: 38 00 ff ff li r0,-1
244: 7c 84 1a 14 add r4,r4,r3
248: 78 00 00 20 clrldi r0,r0,32
24c: 78 89 00 22 rldicl r9,r4,32,32
250: 7c 80 00 38 and r0,r4,r0
254: 7c 09 02 14 add r0,r9,r0
258: 78 09 00 22 rldicl r9,r0,32,32
25c: 7c 00 4a 14 add r0,r0,r9
260: 78 03 00 20 clrldi r3,r0,32
264: 4e 80 00 20 blr
In comparison, the generic implementation of csum_add() gives:
0000000000000290 <.csum_add>:
290: 7c 63 22 14 add r3,r3,r4
294: 7f 83 20 40 cmplw cr7,r3,r4
298: 7c 10 10 26 mfocrf r0,1
29c: 54 00 ef fe rlwinm r0,r0,29,31,31
2a0: 7c 60 1a 14 add r3,r0,r3
2a4: 78 63 00 20 clrldi r3,r3,32
2a8: 4e 80 00 20 blr
And the reverted implementation for PPC64 gives:
0000000000000240 <.csum_add>:
240: 7c 84 1a 14 add r4,r4,r3
244: 78 80 00 22 rldicl r0,r4,32,32
248: 7c 80 22 14 add r4,r0,r4
24c: 78 83 00 20 clrldi r3,r4,32
250: 4e 80 00 20 blr
Fixes: 6ad966d7303b7 ("powerpc/64: Fix checksum folding in csum_add()")
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Acked-by: Paul Mackerras <paulus@ozlabs.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Acked-by: Michal Suchanek <msuchanek@suse.de>
---
arch/powerpc/include/asm/checksum.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/arch/powerpc/include/asm/checksum.h b/arch/powerpc/include/asm/checksum.h
index 842124b199b5..4e63787dc3be 100644
--- a/arch/powerpc/include/asm/checksum.h
+++ b/arch/powerpc/include/asm/checksum.h
@@ -112,7 +112,7 @@ static inline __wsum csum_add(__wsum csum, __wsum addend)
#ifdef __powerpc64__
res += (__force u64)addend;
- return (__force __wsum) from64to32(res);
+ return (__force __wsum)((u32)res + (res >> 32));
#else
asm("addc %0,%0,%1;"
"addze %0,%0;"
--
2.19.2