Similar to reception, raw1394 maintains a fixed-size ringbuffer of packets in kernel memory. The buffer is filled by your application as it queues packets to be sent. The buffer is drained by the hardware driver as it transmits packets on the 1394 bus.
The isochronous transmit handler you provided will be called
from raw1394_loop_iterate() whenever
there is space in the buffer to queue another packet. The
handler is passed a pointer to the first byte of the buffer
space for the packet's data payload, pointers to words
containing the data length in bytes (not counting the
isochronous header), "tag" and "sy" fields, and the
isochronous cycle number at which this packet will be
transmitted. The handler must write the packet's data
payload into the supplied buffer space, and set the values
pointed to by "len", "tag", and "sy" to the appropriate
values. The handler is permitted to write any number of data
bytes, up and including to the value of
max_packet_size passed to
raw1394_iso_xmit_init().
Note: If you passed -1 as the starting cycle to
raw1394_iso_xmit_init(), the cycle
number provided to your handler will be incorrect until after
one buffer's worth of packets have been transmitted.
The transmit handler is also passed a "packet(s) dropped" flag. If this flag is nonzero, it means that one or more outgoing packets have been dropped since the last call to your handler (usually this is because the kernel buffer has gone completely empty or a bus reset has occurred).