ColdFire/68K Microcontrollers and ProcessorsのトピックRe: Address hashing in FEC

Address hashing in FEC

ynaught — Fri, 17 Nov 2006 23:59:44 GMT

I'm now initializing the FEC in my MCF5270 and am a bit puzzled by the Hash tables' configuration.

I think the explanation is clear enough, but am I correct in assuming that the table of MAC addresses and their coresponding hashes (Table 19-36, p. 19-43, MCF5271RM) is totally without value to me, except perhaps for validating my own algorithm?

As I read it, the only way to correctly configure this table is to write my own CRC32 algorithm, just to generate the 6-bit hash. Is there some way I can leverage the hardware CRC generator for this?

TIA

Re: Address hashing in FEC

mnorman — Tue, 21 Nov 2006 02:53:29 GMT

You do have to calculate the hash value in software. Here is a routine to do it:

/********************************************************************/
/*
* Generate the hash table settings for the given address
*
* Parameters:
* addr    48-bit (6 byte) Address to generate the hash for
*
* Return Value:
* The 6 most significant bits of the 32-bit CRC result
*/
uint8
fec_hash_address(const uint8* addr)
{
    uint32 crc;
    uint8 byte;
    int i, j;

    crc = 0xFFFFFFFF;
    for(i=0; i<6; ++i)
    {
        byte = addr[i];
        for(j=0; j<8; ++j)
        {
            if((byte & 0x01)^(crc & 0x01))
            {
                crc >>= 1;
                crc = crc ^ 0xEDB88320;
            }
            else
                crc >>= 1;
            byte >>= 1;
        }
    }
    return (uint8)(crc >> 26);
}
/********************************************************************/

-mnorman

Re: Address hashing in FEC

ynaught — Tue, 21 Nov 2006 04:21:27 GMT

That one works, when none of the others I tried did. All the algorithms I'd found used 0x04C11DB7, but I guess that'd be the MSB-first version...

Thank you!