Hi Tom,

Tom Evans wrote:

UDP Checksumming is enabled with the configuration variable CONFIG_UDP_CHECKSUM. This is only enabled in 7 out of 437 include/configs/*.h files. Enabling UDP checksumming can be useful, as it allows any errors (in downloading via TFTP) to be retried rather than resulting in an image CRC error when the whole download has finished. The download isn't meant to fail, but I'm seeing intermittent corrupted downloads currently.

If CONFIG_UDP_CHECKSUM is enabled on an ARM core, and the CPU is prior to ARM Core Version 6, all packets are reported as having checksum errors.

This is due to the ARM's 32-bit alignment requirements and the following four lines of code in net/net.c::NetReceive()

 xsum += (ntohl(ip->ip_src) >> 16) & 0x0000ffff;
 xsum += (ntohl(ip->ip_src) >>  0) & 0x0000ffff;
 xsum += (ntohl(ip->ip_dst) >> 16) & 0x0000ffff;
 xsum += (ntohl(ip->ip_dst) >>  0) & 0x0000ffff;

The original Ethernet packet is (usually, this is not controlled) 32-bit aligned, the Ethernet header is 14 bytes long, so by design the aligned fields in the IP (and other) headers are all misaligned, at least with the NE2000 driver we're using.

ARM (prior to V6) "silently corrupts" misaligned reads. For 32-bit reads offset by 16 bits it reads 32-bits from the aligned address two bytes below that requested, and then swaps the words.

If the above lines are changed to match other related ARM-related modifications as follows, then the UDP checksum code works:

 tmp = NetReadIP(&ip->ip_src);
 xsum += (ntohl(tmp) >> 16) & 0x0000ffff;
 xsum += (ntohl(tmp) >>  0) & 0x0000ffff;
 tmp = NetReadIP(&ip->ip_dst);
 xsum += (ntohl(tmp) >> 16) & 0x0000ffff;
 xsum += (ntohl(tmp) >>  0) & 0x0000ffff;

I'm afraid I can't generate a patch to do this. Could someone else please incorporate this change if required?

Everything seems logical until this point. Why can't you create a patch?

regards, Ben