Re: [dpdk-dev] [PATCH v3 1/2] ethdev: add capability control API

[Thomas Monjalon <thomas.monjalon@6wind.com>]

2017-03-06 16:35, Dumitrescu, Cristian:
> Hi Thomas,
> 
> Thanks for reviewing this proposal.
> 
> 
> > > Signed-off-by: Cristian Dumitrescu <cristian.dumitrescu@intel.com>
> > > Acked-by: Keith Wiles <keith.wiles@intel.com>
> > > Acked-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>
> > > Acked-by: Hemant Agrawal <hemant.agrawal@nxp.com>
> > [...]
> > > +enum rte_eth_capability {
> > > +	RTE_ETH_CAPABILITY_FLOW = 0, /**< Flow */
> > > +	RTE_ETH_CAPABILITY_TM, /**< Traffic Manager */
> > > +	RTE_ETH_CAPABILITY_MAX
> > > +};
> > [...]
> > >  /**
> > > + * Take capability operations on an Ethernet device.
> > > + *
> > > + * @param port_id
> > > + *   The port identifier of the Ethernet device.
> > > + * @param cap
> > > + *   The capability of the Ethernet device
> > > + * @param arg
> > > + *   A pointer to arguments defined specifically for the operation.
> > > + * @return
> > > + *   - (0) if successful.
> > > + *   - (-ENOTSUP) if hardware doesn't support.
> > > + *   - (-ENODEV) if *port_id* invalid.
> > > + */
> > > +int rte_eth_dev_capability_ops_get(uint8_t port_id,
> > > +	enum rte_eth_capability cap, void *arg);
> > 
> > What is the benefit of getting different kind of capabilities with
> > the same function?
> > enum + void* = ioctl
> > A self-explanatory API should have a dedicated function for each kind
> > of features with different argument types.
> 
> The advantage is providing a standard interface to query the capabilities of the device rather than having each capability provide its own mechanism in a slightly different way.
> 
> IMO this mechanism is of great help to guide the developers of future ethdev features on the clean path to add new features in a modular way, extending the ethdev functionality while doing so in a separate name space and file (that's why I tend to call this a plugin-like mechanism), as opposed to the current monolithic approach for ethdev, where we have 100+ API functions in a single name space and that are split into functional groups just by blank lines in the header file. It is simply the generalization of the mechanism introduced by rte_flow in release 17.02 (so all the credit should go to Adrien and not me).
> 
> IMO, having a standard function as above it cleaner than having a separate and slightly different function per feature. People can quickly see the set of standard ethdev capabilities and which ones are supported by a specific device. Between A) and B) below, I definitely prefer A):
> A) status = rte_eth_dev_capability_ops_get(port_id, RTE_ETH_CABABILITY_TM, &tm_ops);
> B) status = rte_eth_dev_tm_ops_get(port_id, &tm_ops);

I prefer B because instead of tm_ops, you can use some specific tm arguments,
show their types and properly document each parameter.