Re: [dpdk-dev] [RFC PATCH v1 1/4] ethdev: add support for PMD-tuned Tx/Rx parameters

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

> -----Original Message-----
> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Ferruh Yigit
> Sent: Wednesday, March 14, 2018 5:52 PM
> To: Shreyansh Jain <shreyansh.jain@nxp.com>; Horton, Remy <remy.horton@intel.com>; dev@dpdk.org
> Cc: Lu, Wenzhuo <wenzhuo.lu@intel.com>; Wu, Jingjing <jingjing.wu@intel.com>; Zhang, Qi Z <qi.z.zhang@intel.com>; Xing, Beilei
> <beilei.xing@intel.com>; Thomas Monjalon <thomas@monjalon.net>
> Subject: Re: [dpdk-dev] [RFC PATCH v1 1/4] ethdev: add support for PMD-tuned Tx/Rx parameters
> 
> On 3/14/2018 5:23 PM, Shreyansh Jain wrote:
> >> -----Original Message-----
> >> From: Ferruh Yigit [mailto:ferruh.yigit@intel.com]
> >> Sent: Wednesday, March 14, 2018 10:13 PM
> >> To: Remy Horton <remy.horton@intel.com>; dev@dpdk.org
> >> Cc: Wenzhuo Lu <wenzhuo.lu@intel.com>; Jingjing Wu
> >> <jingjing.wu@intel.com>; Qi Zhang <qi.z.zhang@intel.com>; Beilei Xing
> >> <beilei.xing@intel.com>; Shreyansh Jain <shreyansh.jain@nxp.com>;
> >> Thomas Monjalon <thomas@monjalon.net>
> >> Subject: Re: [dpdk-dev] [RFC PATCH v1 1/4] ethdev: add support for PMD-
> >> tuned Tx/Rx parameters
> >>
> >> On 3/14/2018 3:48 PM, Remy Horton wrote:
> >>>
> >>> On 14/03/2018 14:43, Ferruh Yigit wrote:
> >>> [..]
> >>>>>  lib/librte_ether/rte_ethdev.c | 18 ++++++++++++++++++
> >>>>>  lib/librte_ether/rte_ethdev.h | 15 +++++++++++++++
> >>>>
> >>>> Can you please remove deprecation notice in this patch.
> >>>
> >>> Done.
> >>>
> >>>>> +	/* Defaults for drivers that don't implement preferred
> >>>>> +	 * queue parameters.
> >>> [..]
> >>>> Not sure about having these defaults here. It is OK to have defaults
> >> in driver,
> >>>> in application or in config file, but I am not sure if putting them
> >> into device
> >>>> abstraction layer hides them.
> >>>>
> >>>> What about not providing any default in ethdev layer, and get zero
> >> as invalid
> >>>> when using them?
> >>>
> >>> This is something I have been thinking about, and I am going to
> >> remove
> >>> them for the V2. Original motive was to avoid breaking testpmd for
> >> PMDs
> >>> that don't give defaults (i.e. almost all of them). The alternative
> >> is
> >>> to put place-holders into all the PMDs themselves, but I am not sure
> >> if
> >>> this is appropriate.
> >>
> >> I think preferred values should be optional, PMD should have right to
> >> not
> >> provide any. Implementation in 4/4 forces preferred values, either in
> >> all PMDs
> >> or in ethdev layer.
> >>
> >> What about changing approach in application:
> >>  is preferred value provided [1] ?
> >>   yes => use it by sending value 0
> >>   no => use application provided value, same as now, so control should
> >> be in
> >> application.
> >>
> >> I am aware this breaks the comfort of just providing 0 and PMD values
> >> will be
> >> used but covers the case there is no PMD values.
> >>
> >> [1]
> >> it can be possible to check if preferred value provided by comparing 0,
> >> but if 0
> >> is a valid value that can be problem. It may not be problem with
> >> current
> >> variables but it may be when this struct extended, it may be good to
> >> think about
> >> alternative here.
> >
> > I don't think we should use the condition of "yes => use it by sending value 0". That is non-intuitive. Ideally, the application should query
> and then if query responds with value as '0' (which can be valid for some variables in future), it sends its own value to setup functions
> (whether '0' or something else, in case of 0 response, would depend on the knob).
> 
> Right, at that stage application already knows what is the preferred value and
> can directly use it.
> 
> 
> Will it be too much to:
> 
> 1) Adding a new field into "rte_eth_[rt]xconf" to say if exists prefer PMD
> values. "prefer_device_values" ?
> Application can provide values as usual, but if that field is set, abstraction
> layer overwrites the application values with PMD preferred ones. If there is no
> PMD preferred values continue using application ones.
> 
> 
> 2) Add a bitwise "is_set" field to new "preferred_size" struct, which may show
> status of other fields in the struct, if PMD set a valid value for them or not,
> so won't have to rely on the 0 check.

That all seems like too much hassle for such small thing.
If we really want to allow PMD not to provide preferred values -
then instead of adding rte_eth_dev_pref_info into dev_info we can simply
introduce a new optional ethdev API call:
rte_eth_get_pref_params() or so.
If the PMD doesn’t want to provide preferred params to the user it simply
wouldn't implement that function. 

Konstantin

> 
> >
> > Existing example applications should be changed for this. It is tedious, but gives a true example usage.
> 
> Applications already needs to be updated to use this, important part is
> modification is optional.
> 
> >