Re: [dpdk-dev] [RFC 19.11 v2 1/3] ethdev: hide key ethdev structures from public API

["Zapolski, MarcinX A" <marcinx.a.zapolski@intel.com>]

> -----Original Message-----
> From: Bruce Richardson [mailto:bruce.richardson@intel.com]
> Sent: Tuesday, September 10, 2019 2:20 PM
> To: Zapolski, MarcinX A <marcinx.a.zapolski@intel.com>
> Cc: dev@dpdk.org
> Subject: Re: [dpdk-dev] [RFC 19.11 v2 1/3] ethdev: hide key ethdev
> structures from public API
> 
> On Tue, Sep 10, 2019 at 11:13:50AM +0100, Zapolski, MarcinX A wrote:
> > > -----Original Message-----
> > > From: Bruce Richardson [mailto:bruce.richardson@intel.com]
> > > Sent: Tuesday, September 10, 2019 12:07 PM
> > > To: Zapolski, MarcinX A <marcinx.a.zapolski@intel.com>
> > > Cc: dev@dpdk.org
> > > Subject: Re: [dpdk-dev] [RFC 19.11 v2 1/3] ethdev: hide key ethdev
> > > structures from public API
> > >
> > > On Tue, Sep 10, 2019 at 09:59:51AM +0000, Zapolski, MarcinX A wrote:
> > > > <...>
> > > > > @@ -3994,7 +4054,55 @@ void *
> > > > >  rte_eth_dev_get_sec_ctx(uint16_t port_id);
> > > > >
> > > > >
> > > > > -#include <rte_ethdev_core.h>
> > > > > +struct rte_eth_dev_callback;
> > > > > +/** Structure to keep track of registered callbacks */
> > > > > +TAILQ_HEAD(rte_eth_dev_cb_list, rte_eth_dev_callback);
> > > > > +
> > > > > +/**
> > > > > + * Structure used to hold information about the callbacks to be
> > > > > +called for a
> > > > > + * queue on RX and TX.
> > > > > + */
> > > > > +struct rte_eth_rxtx_callback {
> > > > > +	struct rte_eth_rxtx_callback *next;
> > > > > +	union{
> > > > > +		rte_rx_callback_fn rx;
> > > > > +		rte_tx_callback_fn tx;
> > > > > +	} fn;
> > > > > +	void *param;
> > > > > +};
> > > > > +
> > > > > +/**
> > > > > + * The generic data structure associated with each ethernet device.
> > > > > + *
> > > > > + * Pointers to burst-oriented packet receive and transmit
> > > > > +functions are
> > > > > + * located at the beginning of the structure, along with the
> > > > > +pointer to
> > > > > + * where all the data elements for the particular device are
> > > > > +stored in shared
> > > > > + * memory. This split allows the function pointer and driver
> > > > > +data to be per-
> > > > > + * process, while the actual configuration data for the device is
> shared.
> > > > > + */
> > > > I would like to raise one more concern (maybe not the smartest
> > > > thing to do involving my own patch, but I will sleep better when this is
> resolved).
> > > > Does anyone know what the author of this comment had in mind? I
> > > > could not find any code that utilizes this split, and this patch
> > > > clearly breaks it. If it is obsolete, I guess this comment could be
> removed.
> > >
> > > As the comment suggest the split is for multi-process suport. The
> > > data structures for the RX/TX queues etc. all need to be common
> > > across processes, while the function pointers need to be
> > > per-process. Originally, in very early versions of DPDK there was no
> > > rte_eth_dev_data structure, all fields - both data and function pointers -
> were in rte_eth_dev itself.
> > >
> > > /Bruce
> >
> > Yes, I see the need to have the elements separated. But the comment
> > suggests that the order of variables in structure is important. I just
> > want to make sure that it is not.
> 
> I don't believe it's particularly significant. The main thing was to ensure that
> we kept the fast-path accessed data on the same cacheline.

Thank you for clarifying.