The problem isn’t with the LwIP code, but with the Ethernet hardware driver
(hdkif.c: hdkif_output). The driver ensures that packets are always at least
60 bytes long. The implementation I have (from TI for the Hercules Device –
RM46L852) has this code:
/* adjust the packet length if less than minimum required */
if(p->tot_len < MIN_PKT_LEN) {
p->tot_len = MIN_PKT_LEN;
p->len = MIN_PKT_LEN;
}
This works as long as buffers are not chained. When the buffers are chained,
the packet is permanently destroyed so re-transmits will always fail. The fix
is to change the above code to:
/* adjust the packet length if less than minimum required */
if(p->tot_len < MIN_PKT_LEN) {
struct pbuf *temp_p;
int diff_len = MIN_PKT_LEN - p->tot_len;
for (temp_p = p; temp_p->next != NULL; temp_p = temp_p->next)
temp_p->tot_len += diff_len;
temp_p->tot_len += diff_len;
temp_p->len += diff_len;
}
I realize this isn’t an LwIP problem, but I bet a lot of people have this
problem waiting to happen as soon as a short chained packet is sent.
-Jeremy
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