parisc: Fix ip_fast_csum

IP checksum unit tests report the following error when run on hppa/hppa64.

    # test_ip_fast_csum: ASSERTION FAILED at lib/checksum_kunit.c:463
    Expected ( u64)csum_result == ( u64)expected, but
        ( u64)csum_result == 33754 (0x83da)
        ( u64)expected == 10946 (0x2ac2)
    not ok 4 test_ip_fast_csum

0x83da is the expected result if the IP header length is 20 bytes. 0x2ac2
is the expected result if the IP header length is 24 bytes. The test fails
with an IP header length of 24 bytes. It appears that ip_fast_csum()
always returns the checksum for a 20-byte header, no matter how long
the header actually is.

Code analysis shows a suspicious assembler sequence in ip_fast_csum().

 "      addc            %0, %3, %0\n"
 "1:    ldws,ma         4(%1), %3\n"
 "      addib,<         0, %2, 1b\n"	<---

While my understanding of HPPA assembler is limited, it does not seem
to make much sense to subtract 0 from a register and to expect the result
to ever be negative. Subtracting 1 from the length parameter makes more
sense. On top of that, the operation should be repeated if and only if
the result is still > 0, so change the suspicious instruction to
 "      addib,>         -1, %2, 1b\n"

The IP checksum unit test passes after this change.

Cc: Palmer Dabbelt <palmer@rivosinc.com>
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Charlie Jenkins <charlie@rivosinc.com>
Reviewed-by: Charlie Jenkins <charlie@rivosinc.com>
Signed-off-by: Helge Deller <deller@gmx.de>

Author: groeck

arch/parisc/include/asm/checksum.h

@@ -40,7 +40,7 @@ static inline __sum16 ip_fast_csum(const void *iph, unsigned int ihl)
 "	addc		%0, %5, %0\n"
 "	addc		%0, %3, %0\n"
 "1:	ldws,ma		4(%1), %3\n"
-"	addib,<		0, %2, 1b\n"
+"	addib,>		-1, %2, 1b\n"
 "	addc		%0, %3, %0\n"
 "\n"
 "	extru		%0, 31, 16, %4\n"