firewire: cdev: add PHY pinging

This extends the FW_CDEV_IOC_SEND_PHY_PACKET ioctl() for /dev/fw* to be
useful for ping time measurements.  One application for it would be gap
count optimization in userspace that is based on ping times rather than
hop count.  (The latter is implemented in firewire-core itself but is
not applicable to beta PHYs that act as repeater.)

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>

include/linux/firewire-cdev.h

@@ -294,7 +294,10 @@ struct fw_cdev_event_iso_resource {
  * @length:	Data length in bytes
  * @data:	Incoming data
  *
- * If @type is %FW_CDEV_EVENT_PHY_PACKET_SENT, @length is 0 and @data empty.
+ * If @type is %FW_CDEV_EVENT_PHY_PACKET_SENT, @length is 0 and @data empty,
+ * except in case of a ping packet:  Then, @length is 4, and @data[0] is the
+ * ping time in 49.152MHz clocks if @rcode is %RCODE_COMPLETE.
+ *
  * If @type is %FW_CDEV_EVENT_PHY_PACKET_RECEIVED, @length is 8 and @data
  * consists of the two PHY packet quadlets, in host byte order.
  */