Re: [PATCH 1/3] ethdev: New API to free consumed buffers in TX ring

[Stephen Hemminger <stephen@networkplumber.org>]

On Fri, 16 Dec 2016 07:48:49 -0500
Billy McFall <bmcfall@redhat.com> 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.