Re: [PATCH v6 1/3] ethdev: new API to free consumed buffers in Tx ring

[Olivier Matz <olivier.matz@6wind.com>]

Hi Billy,

On Thu,  9 Mar 2017 15:51:17 -0500, Billy McFall <bmcfall@redhat.com> wrote:
> Add a new API to force free consumed buffers on Tx ring. API will return
> the number of packets freed (0-n) or error code if feature not supported
> (-ENOTSUP) or input invalid (-ENODEV).
> 
> Please double check my python coding in conf.py and make sure I
> implemented 'feature_str_len' properly.
> 
> Signed-off-by: Billy McFall <bmcfall@redhat.com>

[...]

> diff --git a/doc/guides/prog_guide/mempool_lib.rst b/doc/guides/prog_guide/mempool_lib.rst
> index ffdc109..a117881 100644
> --- a/doc/guides/prog_guide/mempool_lib.rst
> +++ b/doc/guides/prog_guide/mempool_lib.rst
> @@ -132,6 +132,32 @@ These user-owned caches can be explicitly passed to ``rte_mempool_generic_put()`
>  The ``rte_mempool_default_cache()`` call returns the default internal cache if any.
>  In contrast to the default caches, user-owned caches can be used by non-EAL threads too.
>  
> +In addition to a core's local cache, many of the drivers don't release the mbuf back to the mempool, or local cache,
> +immediately after the packet has been transmitted.
> +Instead, they leave the mbuf in their Tx ring and either perform a bulk release when the ``tx_rs_thresh`` has been
> +crossed or free the mbuf when a slot in the Tx ring is needed.
> +
> +An application can request the driver to release used mbufs with the ``rte_eth_tx_done_cleanup()`` API.
> +This API requests the driver to release mbufs that are no longer in use, independent of whether or not the
> +``tx_rs_thresh`` has been crossed.
> +There are two scenarios when an application may want the mbuf released immediately:
> +
> +* When a given packet needs to be sent to multiple destination interfaces (either for Layer 2 flooding or Layer 3
> +  multi-cast).
> +  One option is to make a copy of the packet or a copy of the header portion that needs to be manipulated.
> +  A second option is to transmit the packet and then poll the ``rte_eth_tx_done_cleanup()`` API until the reference
> +  count on the packet is decremented.
> +  Then the same packet can be transmitted to the next destination interface.
> +
> +* If an application is designed to make multiple runs, like a packet generator, and one run has completed.
> +  The application may want to reset to a clean state.
> +  In this case, it may want to call the ``rte_eth_tx_done_cleanup()`` API to request each destination interface it has
> +  been using to release all of its used mbufs.
> +
> +To determine if a driver supports this API, check for the *Free Tx mbuf on demand* feature in the *Network Interface
> +Controller Drivers* document.

I'm not sure the mempool documentation is the proper place to document
this API. I'll suggest to move it in ethdev documentation (poll_mode_drv.rst).

You can keep a small paragraph in mempool lib, but keep in mind that
mempool is not necessarily a pool of mbuf (it can be any kind of object).


Thanks,
Olivier