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

[Bruce Richardson]

On Wed, Mar 14, 2018 at 09:02:47PM +0000, Ferruh Yigit wrote:
> On 3/14/2018 6:53 PM, Ananyev, Konstantin wrote:
> > 
> > 
> >> -----Original Message-----
> >> From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Ferruh Yigit
> >> Sent: Wednesday, March 14, 2018 5:52 PM
> >> To: Shreyansh Jain <shreyansh.jain at nxp.com>; Horton, Remy <remy.horton at intel.com>; dev at dpdk.org
> >> Cc: Lu, Wenzhuo <wenzhuo.lu at intel.com>; Wu, Jingjing <jingjing.wu at intel.com>; Zhang, Qi Z <qi.z.zhang at intel.com>; Xing, Beilei
> >> <beilei.xing at intel.com>; Thomas Monjalon <thomas at 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 at intel.com]
> >>>> Sent: Wednesday, March 14, 2018 10:13 PM
> >>>> To: Remy Horton <remy.horton at intel.com>; dev at dpdk.org
> >>>> Cc: Wenzhuo Lu <wenzhuo.lu at intel.com>; Jingjing Wu
> >>>> <jingjing.wu at intel.com>; Qi Zhang <qi.z.zhang at intel.com>; Beilei Xing
> >>>> <beilei.xing at intel.com>; Shreyansh Jain <shreyansh.jain at nxp.com>;
> >>>> Thomas Monjalon <thomas at 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.
> 
> Fair enough.
> 
> > 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. 
> 
> Same can be done with updated rte_eth_dev_info.
> Only application needs to check and use PMD preferred values, so this will mean
> dropping "pass 0 to get preferred values" feature in initial set.
> 
> > 
I actually don't see the issue with having ethdev provide reasonable
default values. If those don't work for a driver, then let the driver
provide it's own values. If the defaults don't work for an app, then let
the app override the provided values.

It really is going to make the app writers job easier if we do things this
way. The only thing you are missing is the info as to whether it's ethdev
or the driver that's providing the values, but in the case that it's
ethdev, then the driver by definition "doesn't care", so we can treat them
as driver provided values. What's the downside?

/Bruce