On Fri, 16 Dec 2016 07:48:49 -0500
Billy McFall [off-list ref] wrote:

/**
+ * Request the driver to free mbufs currently cached by the driver. The
+ * driver will only free the mbuf if it is no longer in use.
+ *
+ * @param port_id
+ *   The port identifier of the Ethernet device.
+ * @param queue_id
+ *   The index of the transmit queue through which output packets must be
+ *   sent.
+ *   The value must be in the range [0, nb_tx_queue - 1] previously supplied
+ *   to rte_eth_dev_configure().
+ * @param free_cnt
+ *   Maximum number of packets to free. Use 0 to indicate all possible packets
+ *   should be freed. Note that a packet may be using multiple mbufs.
+ * @param buffer
+ *   Buffer used to collect packets to be sent. If provided, the buffer will
+ *   be flushed, even if the current length is less than buffer->size. Pass NULL
+ *   if buffer has already been flushed.
+ * @param sent
+ *   Pointer to return number of packets sent if buffer has packets to be sent.
+ *   If *buffer is supplied, *sent must also be supplied.
+ * @return
+ *   Failure: < 0
+ *     -ENODEV: Invalid interface
+ *     -ENOTSUP: Driver does not support function
+ *   Success: >= 0
+ *     0-n: Number of packets freed. More packets may still remain in ring that
+ *     are in use.
+ */
+
+static inline int
+rte_eth_tx_done_cleanup(uint8_t port_id, uint16_t queue_id,  uint32_t free_cnt,
+		struct rte_eth_dev_tx_buffer *buffer, uint16_t *sent)

This API is more complex than it needs to be.
For the typical use case of OOM kind of cleanup, this is overkill.
There is no need for:
  free_cnt - device driver should just free all
  buffer/param - the application should not care.

The DPDK model is that once mbuf's are passed to device, the device "owns"
the mbuf. I think changing that model is just going to break things for
no gain.  It does make sense to have a "please cleanup your mbufs" call.
If application is using special mbuf's then it can use the normal callback
on done model.