Em Fri, Oct 31, 2003 at 11:13:01PM -0200, Arnaldo C. Melo escreveu:
> Em Fri, Oct 31, 2003 at 04:38:43PM -0800, Stephen Hemminger escreveu:
> >
> > 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).
>
> Testing...
[root@lolo apple]# md5sum b/kernel-2.4.21-32898cl.i586.rpm
kernel-2.4.21-32898cl.i586.rpm
79adde6c4dd97fb214d30009e100a835 b/kernel-2.4.21-32898cl.i586.rpm
79adde6c4dd97fb214d30009e100a835 kernel-2.4.21-32898cl.i586.rpm
[root@lolo apple]#
Perfect, it works as well, but as this is not bugfixing, but an improvement,
I'd say this can well wait for 2.6.1 :-)
Case closed.
- Arnaldo
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