I decided to look at adding DMA support for the Phytec MPC5554 for the SMC91111
to speed up network performance. It's a twisty piece of work. So far I've
only looked at where the info in the receive FIFO is copied into the mbuf.
Here's the critical loop (around line 215). LAN91CXX_32BIT_RX is defined,
meaning "sizeof(rxdt)" and "sizeof(*data)" are 2:
while (mlen >= sizeof(*data)) {
#ifdef LAN91CXX_32BIT_RX
rxd_t val = get_data(cpd);
*(unsigned short *)data = (val >> 16) & 0xffff;
data = (rxd_t *)((unsigned short *)data + 1);
*(unsigned short *)data = (val & 0xffff);
data = (rxd_t *)((unsigned short *)data + 1);
#else
*data++ = get_data(cpd);
#endif
mlen -= sizeof(*data);
plen -= sizeof(*data);
}
Let's simplify this for the case of sizeof(rxd_t) == 4:
while (mlen >= sizeof(rxd_t)) {
rxd_t val = get_data(cpd);
((uint16_t *)data)[0] = val >> 16;
((uint16_t *)data)[1] = val;
data++;
mlen -= sizeof(rxd_t);
plen -= sizeof(rxd_t);
}
What is this trying to do? I think on a big endian system it's (almost, other
than the extra work) a NOP and on a little endian system it's swapping the two
16 bit halves of the word, but it's not swapping the bytes. I don't understand.
Maybe this is only in use on big endian systems? I'm using it on PowerPC and I
know it's also used on the SPARC.
Peter
-----------------
Peter Dufault
HD Associates, Inc. Software and System Engineering
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