[U-Boot] [PATCH]Fix checksum to handle odd-length packet

[Wolfgang Denk]

Dear Joakim Tjernlund,

In message <OFDFF8A95F.374EB267-ONC1257681.0042613C-C1257681.004485DD at transmode.se> you wrote:
>
> > > +       if (len == 1) {
> > > +               xsum += (*p & 0xff00);
> >
> > I doubt that this code is endianess-clean.
> 
> Nope, I would think some thing like this would work better:
>       count = len >> 1; /* div by 2 */
> 	for(p--; count; --count)
> 	  xsum += *++p;
> 
> 	if (len & 1) /* Odd */
> 	  xsum += *(u_char *)(++p);   /* one byte only */

It probably depends on the definition of "better" ;-)

Running this test code:

-----------------------------------------
#include <stdio.h>
#include <string.h>

#define LENGTH	6

int main (void)
{
	char string[LENGTH] = { 1, 2, 3, 4, 5, 0, };
	unsigned short array[LENGTH/2];
	unsigned short *p;
	unsigned short xsum, xsum1, xsum2;
	int i, count;

	memcpy (array, string, LENGTH);

	count = LENGTH / 2;

	xsum = 0;
	p = array;

	while (count > 1) {
		printf ("Adding 0x%04x\n", *p);
		xsum += *p++;
		--count;
	}

	xsum1 = xsum + (*p & 0xff00);

	xsum2 = xsum + *(unsigned char *)(p);

	printf ("*p = 0x%04x\n", *p);
	printf ("xsum = %04x,  xsum1 = %04x,  xsum2 = %04x\n",
		xsum, xsum1, xsum2);
	
	return (0);
}
-----------------------------------------

on a little endian system gives:

	-> ./f-le
	Adding 0x0201
	Adding 0x0403
	*p = 0x0005
	xsum = 0604,  xsum1 = 0604,  xsum2 = 0609

while running it on a big endian system gives

	-> ./f-be
	Adding 0x0102
	Adding 0x0304
	*p = 0x0500
	xsum = 0406,  xsum1 = 0906,  xsum2 = 040b

I don't know what you consider to be the exact result,  but  fact  is
that  even if we ignore byte swapping, none of the implementations is
endianess clean.

Of course, there is a chance that I mis-implemented your suggestion.

Best regards,

Wolfgang Denk

-- 
DENX Software Engineering GmbH,     MD: Wolfgang Denk & Detlev Zundel
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany
Phone: (+49)-8142-66989-10 Fax: (+49)-8142-66989-80 Email: wd at denx.de
       There is enough for the need of everyone in this world,
       but not for the greed of everyone.     - Mahatma Gandhi