powerpc: add support for csum_add()

The C version of csum_add() as defined in include/net/checksum.h gives
the following assembly in ppc32:
0: 7c 04 1a 14 add r0,r4,r3
4: 7c 64 00 10 subfc r3,r4,r0
8: 7c 63 19 10 subfe r3,r3,r3
c: 7c 63 00 50 subf r3,r3,r0
and the following in ppc64:
0xc000000000001af8 <+0>: add r3,r3,r4
0xc000000000001afc <+4>: cmplw cr7,r3,r4
0xc000000000001b00 <+8>: mfcr r4
0xc000000000001b04 <+12>: rlwinm r4,r4,29,31,31
0xc000000000001b08 <+16>: add r3,r4,r3
0xc000000000001b0c <+20>: clrldi r3,r3,32
0xc000000000001b10 <+24>: blr

include/net/checksum.h also offers the possibility to define an arch
specific function. This patch provides a specific csum_add() inline
function.

Signed-off-by: Christophe Leroy <[e-mail]>
Signed-off-by: Scott Wood <[e-mail]>

arch/powerpc/include/asm/checksum.h

···
 	return csum_fold(csum_tcpudp_nofold(saddr, daddr, len, proto, sum));
 }
 
+#define HAVE_ARCH_CSUM_ADD
+static inline __wsum csum_add(__wsum csum, __wsum addend)
+{
+#ifdef __powerpc64__
+	u64 res = (__force u64)csum;
+
+	res += (__force u64)addend;
+	return (__force __wsum)((u32)res + (res >> 32));
+#else
+	asm("addc %0,%0,%1;"
+	    "addze %0,%0;"
+	    : "+r" (csum) : "r" (addend));
+	return csum;
+#endif
+}
+
 #endif
 #endif /* __KERNEL__ */
 #endif