From mboxrd@z Thu Jan  1 00:00:00 1970
From: Thomas Monjalon <thomas.monjalon@6wind.com>
Subject: Re: [PATCH v9 2/8] ethdev: add new api to add Rx
	callback as head of the list
Date: Tue, 14 Jun 2016 22:01:48 +0200
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To: Reshma Pattan <reshma.pattan@intel.com>
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2016-06-14 10:38, Reshma Pattan:
> Added new public api rte_eth_add_first_rx_callback to add given
> callback as head of the list.
> 
> Signed-off-by: Reshma Pattan <reshma.pattan@intel.com>
> ---
> +/*
> +* Add a callback that must be called first on packet RX on a given port
> +* and queue.
> +*
> +* This API configures a first function to be called for each burst of
> +* packets received on a given NIC port queue. The return value is a pointer
> +* that can be used to later remove the callback using
> +* rte_eth_remove_rx_callback().
> +*
> +* Multiple functions are called in the order that they are added.
> +*
> +* @param port_id
> +*   The port identifier of the Ethernet device.
> +* @param queue_id
> +*   The queue on the Ethernet device on which the callback is to be added.
> +* @param fn
> +*   The callback function
> +* @param user_param
> +*   A generic pointer parameter which will be passed to each invocation of the
> +*   callback function on this port and queue.
> +*
> +* @return
> +*   NULL on error.
> +*   On success, a pointer value which can later be used to remove the callback.
> +*/
> +void *rte_eth_add_first_rx_callback(uint8_t port_id, uint16_t queue_id,
> +		rte_rx_callback_fn fn, void *user_param);

Sorry I fail to understand why this function is needed.
What cannot be done in rte_eth_add_rx_callback?