Re: [IPX]: Fix checksum computation.

[Stephen Hemminger <shemminger@osdl.org>]

Okay, here is the standard: (Inside Appletalk)

> The DDP checksum is provided to detect errors caused by faulty operation (such as memor
> data bus errors) within routers on the internet. Implementers of DDP should treat generati
> the checksum as an optional feature. The 16-bit DDP checksum is computed as follows:
> CkSum := 0 ;
> FOR each datagram byte starting with the byte immediately following th
> Checksum field
> REPEAT the following algorithm:
>           CkSum := CkSum + byte; (unsigned addition)
>           Rotate CkSum left one bit, rotating the most significant bit in
>                               least significant bit;
> IF, at the end, CkSum = 0 THEN
>           CkSum := $FFFF (all ones).
> Reception of a datagram with CkSum equal to 0 implies that a checksum is not performed.


Here is the old loop:

	while (len--) {
		sum += *data;
		sum <<=1;
		if (sum & 0x10000) {
			sum++;
			sum &= 0xffff;
		}
		data++;
	}

My buggy loop is:

	while (len--) {
		sum += *data++;
		sum <<= 1;
		sum = ((sum >> 16) + sum) & 0xFFFF;
	}

The problem is the carry from the first addition needs to be dropped
not folded back (like IP).  

Corrected fast code is:

	while (len--) {
		sum += *data++;
		sum <<= 1;
		sum = (((sum & 0x10000) >> 16) + sum) & 0xffff;
	}

At least it is correct on the standalone random data test, and the
new code is 30% faster for the cached memory case (13.7 clks/byte vs 18 clks/byte).