Re: [dpdk-dev] [PATCH v2 3/3] ethdev: improve flow mark Rx offload deprecation notice

[Andrew Rybchenko <arybchenko@solarflare.com>]

On 11/22/19 2:15 PM, Thomas Monjalon wrote:
> 22/11/2019 11:12, Andrew Rybchenko:
>> On 11/22/19 1:01 AM, Thomas Monjalon wrote:
>>> 19/11/2019 13:12, Andrew Rybchenko:
>>>> The deprecation notice is required since it adds more requirements
>>>> when RTE flow mark and flag actions may be used and require
>>>> changes in applications.
>>> I am still not sure what is the best solution here.
>>> I continued to think about it in this thread:
>>> 	http://mails.dpdk.org/archives/dev/2019-November/151960.html
>>>
>>> I think we cannot require any application change until 20.11
>>> in order to keep API (and behaviour) compatibility.
>> Expected, but still very disappointing.
>>
>> The feature is implemented by Pavan (@ Marvell), supported by me,
>> used by Qi (@ Intel), looks better than alternatives from application
>> developer point of view [1] and finally postponed for 1 year without really
>> strong motivation.
> I see different valuable point of views. This is enough motivation.

It looks like I miss it in previous discussion, I would be thankful if
you give me links to read or hints how to find.

> And no, it is not postponed by one year.
> Next release can implement a new API.
>
>> I disagree that it is tightly related to moving
>> mark/flag to
>> dynamic field/flag and absolutely blocked by it. Yes, I know that the are
>> concerns from the very beginning, but the problem is explained [2] and clear
>> and no full-featured alternative solution is suggested. Solution suggested
>> by Ori has many significant drawbacks as explained in [2] and highlighted
>> in further discussion.
> I disagree with working only on mark action while there are a lot
> of other configs which have to be implemented in drivers.
>
> The reality is that some drivers decided to have some "optimizations"
> disabling some features, and you want the application to opt-in
> in order to allow your optimized paths.

Strictly speaking it is not about driver optimized paths only, but HW
configuration as well which can be done on start-up only (not dynamic) and
could be per-queue in fact.

> Note that opt-in is different of really enabling an offload.
> For some basic port-level features like RSS hash,
> it is enabled with an offload flag before starting the port,
> acting as an opt-in.

Could you highlight the difference between opt-in and offload.
What is the key difference which makes one solution better
than another? Why different mechanism is required?

> Some features have some dedicated API, which may be enabled after
> starting the port, and no way to opt-in (or opt-out) before start.

It sounds like you have examples in your mind. Please, share.

> A lot of features are using rte_flow API which is in this situation.
> If we take the opt-in path, let's not do it only for the mark action,
> but let's create a real API for it:
> 	rte_eth_dev_optin()
> 	rte_eth_dev_optinall()
> 	rte_eth_dev_optoutl()

Introducing new types of controls would make configuration more and
more complex. I think that many different types of control would
over-complicate it. May be it is unavoidable, but it should be clear
why the problem cannot be solved using existing types of controls
(e.g. offloads).

> I think the motivation is strong enough.
>
>> [1] http://inbox.dpdk.org/dev/1573203631946.15959@kth.se/
>> [2]
>> http://inbox.dpdk.org/dev/f170105b-9c60-1b04-cb18-52e0951ddcdb@solarflare.com/
>>
>>> If something would be implemented in 20.02,
>>> it must be a new and optional API.
>> Flow mark and flag may work without the offload with some drivers,
>> but some drivers require the offload to make it work. Flow API error
>> should contain message which says that the offload is disabled and
>> must be enabled.
> Yes, the PMD should return an explicit error about a feature being disabled.
> How does it impact ethdev API?

It is still the offload discussed in the deprecation notice.
The solution is far from ideal, since allows the difference in PMDs
behaviour and an application debugged on one PMD may not
work using another PMD (unfortunately it is true in any case, but
such definition makes it 100% legal).

>>> That's why I think no deprecation notice is required.
>>>
>>> [...]
>>>> +* ethdev: New offload flag ``DEV_RX_OFFLOAD_FLOW_MARK`` will be added in 20.02.
>>>> +  This will provide application an information if ``RTE_FLOW_ACTION_TYPE_MARK``
>>>> +  or ``RTE_FLOW_ACTION_TYPE_FLAG`` is supported and, what is more important,
>>>> +  allow an application to let PMD know that it would like to use these
>>>> +  features.
>>>> +  PMD may use the information to choose optimal datapath implementation and
>>>> +  configure HW appropriately to optimize performance and/or resources usage.