On Fri, Aug 09, 2013 at 10:50:19AM +0200, Uwe Kleine-König wrote: > > D'oh. There's a bug indeed. For a good packet net_checksum returns > > 0xffff (all ones in an u16). So the check should be: > > > > return net_checksum(ptr, len) == 0xffff; > with > > return net_checksum(ptr, len) + 1 > > net_checksum_ok returns always something >0 (i.e. success) because both > summands are converted to unsigned, and so never catches an error[1], > does it? > > > U-Boot has this instead: > > > > return !((net_checksum(ptr, len) + 1) & 0xfffe); > > > > From what I see both above should be equivalent so I wonder why U-Boot > > has such a complicated code here. Some compiler optimization or is this > > something I don't see? > This isn't equivalent. The U-Boot code returns 1 iff net_checksum > returns 0 or 0xffff; 0 otherwise. Hm, indeed. Which brings me to my next question: In which cases does net_checksum() return 0 and the result can be considered ok? Sascha -- Pengutronix e.K. | | Industrial Linux Solutions | http://www.pengutronix.de/ | Peiner Str. 6-8, 31137 Hildesheim, Germany | Phone: +49-5121-206917-0 | Amtsgericht Hildesheim, HRA 2686 | Fax: +49-5121-206917-5555 | _______________________________________________ barebox mailing list barebox@xxxxxxxxxxxxxxxxxxx http://lists.infradead.org/mailman/listinfo/barebox
