2
Copies transmit data from memory to the card using 32-bit writes. Only
a multiple of 4 bytes can be copied this way.
3
If some number of bytes (fewer than 4) remain in the current memory
buffer, the driver either copies those bytes directly to the card (if they
were the last bytes for the entire frame), or combines those bytes with
bytes from the next memory buffer (if there is more data for this frame).
9.2.4 Accounting for Outgoing Bytes
The following code shows how the
el_start_locked( )
routine accounts
for the outgoing bytes:
sc->txfree -= ((m->m_pkthdr.len + 3) &
~ 0x3);
1
sc->txfree -= 4;
1
Maintains the number of bytes free in the transmit FIFO.
9.2.5 Updating Counters, Freeing the Transmit Buffer, and Marking
the Output Process as Active
The following code shows how the
el_start_locked( )
routine updates
counters, frees the transmit buffer, and marks the output process as active:
ADD_XMIT_PACKET(ifp, sc, m->m_pkthdr.len);
1
eh = mtod(m, struct ether_header *);
if (eh->ether_dhost[0] & 0x1) {
ADD_XMIT_MPACKET(ifp, sc, m->m_pkthdr.len);
}
m_freem(m);
2
ifp->if_flags |= IFF_OACTIVE;
} else if (m) {
3
IF_PREPEND(&ifp->if_snd, m);
break;
} else
break;
}
1
Updates the counters using the
ADD_XMIT_PACKET
and possibly the
ADD_XMIT_MPACKET
(for multicast packets) macros. These macros are
defined in the
lan_common.h
file. Most network drivers perform this
task in the transmit complete interface.
2
Calls the
m_freem( )
routine to free the
mbuf
buffer. Network drivers
must free the buffer after the transmit operation is complete.
3
If there is no room for this transmit, puts the
mbuf
back on the queue.
Implementing the Start Section 9–7
