Re: [dpdk-dev] [PATCH v9 2/8] ethdev: add new api to add Rx callback as head of the list

["Pattan, Reshma" <reshma.pattan@intel.com>]

Hi,

> -----Original Message-----
> From: Thomas Monjalon [mailto:thomas.monjalon@6wind.com]
> Sent: Tuesday, June 14, 2016 9:02 PM
> To: Pattan, Reshma <reshma.pattan@intel.com>
> Cc: dev@dpdk.org
> Subject: Re: [dpdk-dev] [PATCH v9 2/8] ethdev: add new api to add Rx callback
> as head of the list
> 
> 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?

Packet capturing framework should display Rx packets of NIC even before they are being processed by other callbacks of the 
application (because other callback s of application may change the packet data as part of the processing).
So packet capturing framework should register  a callback at the head of the Rx callback list so that callback always gets 
called  first before any other callbacks of the applications.  Hence this API is introduced. 

Thanks,
Reshma