Re: [dpdk-dev] [PATCH v4] doc: add new tables for rte flow items and actions support

[Ferruh Yigit <ferruh.yigit@intel.com>]

On 2/18/2021 5:58 PM, Ajit Khaparde wrote:
> On Wed, Feb 17, 2021 at 2:49 AM Thomas Monjalon <thomas@monjalon.net> wrote:
> 
>> 17/02/2021 11:37, Ferruh Yigit:
>>> On 2/17/2021 5:57 AM, Asaf Penso wrote:
>>>> From: Ferruh Yigit <ferruh.yigit@intel.com>
>>>>> On 2/7/2021 10:52 AM, Asaf Penso wrote:
>>>>>> In http://doc.dpdk.org/guides/nics/overview.html, table 1.1 lists
>> all
>>>>>> supported features.
>>>>>> It has a single line for "Flow API" that refers to rte_flow support.
>>>>>> rte_flow is composed of many items and actions that are not expressed
>>>>>> in this single line.
>>>>>>
>>>>>> The following new tables are suggested:
>>>>>> 1. rte_flow items
>>>>>> 2. rte_flow actions
>>>>>>
>>>>>
>>>>> Hi Asaf,
>>>>>
>>>>> I understand the intention, but I am not sure about this.
>>>>>
>>>>> The Flow API does not provide a capability or feature list in the API
>> level, by
>>>>> design, because it is very hard to do it correct, but this patch
>> tries to do it in the
>>>>> documentation level.
>>>>>
>>>>> This will be missing lots of details, the flow items and actions
>> documented as
>>>>> supported may and may not be supported based on the details.
>>>>>
>>>>
>>>> Which missing details are you referring to? All flow items and all
>> actions are listed.
>>>>
>>>
>>> Patterns are complex, any rule can be valid or invalid based on provided
>> pattern
>>> values (details), also any rule can be valid or invalid based on
>> previous rules
>>> or configuration.
>>>
>>> In practice this information is much more useful if it is provided by
>> API, but
>>> we are not able to do it because of its complex nature, it should be
>> same level
>>> of complexity to provide this information by documentation.
>>>
>>>>> It will be very hard to read this table (when it becomes full), also
>> will be very hard
>>>>> to maintain.
>>>>
>>>> As part of any documentation change in rte_flow the developer would
>> also need to update this table.
>>>> Why would it be very hard to maintain?
>>>   >
>>>
>>> Ahh, that sound so simple when you say like this :) In practice even
>> keeping
>>> feature list requiring lots of effort, developers are
>>> missing/neglecting/ignoring updating documentation when updating the
>> code.
>>>
>>> And for this case is partially correct table a useful information? If
>> this is
>>> not completely correct people won't rely on it and it will become just
>> useless.
>>> So this feature should come with an automated way to detect if a rule
>> supported
>>> but not documented, or even better this table should be generated from
>> code
>>> automatically.
>>>
>>>>>
>>>>>
>>>>> Let me start with a question, who do you think will be your consumer?
>>>>> Who will benefit from this table and how?
>>>>
>>>> We get a lot of questions from users regarding rte_flow support and we
>> do not have a single place with proper documentation.
>>>> I can ask the same about the overall feature table, right? There is a
>> value to document the support.
>>>>
>>>
>>> Let's discuss the feature table separately, I think that is a valid
>> question.
>>>
>>> For the rte_flow, who is asking questions? End user, or application
>> developer?
>>> So is this intended to be a marketing documentation or technical
>> documentation?
>>>
>>> And what is the nature of the questions, if it is related to the
>> rte_flow, there
>>> is already a proper documentation for it:
>>> https://doc.dpdk.org/guides/prog_guide/rte_flow.html
>>>
>>> If this question is if any specific rule supported by a specific PMD,
>> right now
>>> only valid way to say this that I am aware of is, run
>> 'rte_flow_validate()' and see.
>>> Not sure if we can document this properly.
>>
>> I think in general we are missing a big disclaimer
>> on top of this overview page:
>>          Each feature may have some hardware limitations.
>>
>> Then there is a need, both for application developers and end-users,
>> to know which feature can be supported by a PMD,
>> or which PMD can support a feature.
>> Yes there are complex limitations with hardware offloads in general.
>> Yes it would be nice to report some tested capabilities with a CI.
>> But it does not mean we should not try to document it in my opinion.
>>
> +1 to all of these.
> A document like this will help give an idea on what is possible with the
> PMD without looking at the code. Beyond that, the user can check with
> the vendor/developer for specific details if needed.
> 

I am still feeling we are trying to workaround flow API design constrain with 
documentation, although we know it won't be complete.
And not really clear who will benefit from this in what way.

Anyway, as mentioned above I am concerned the maintenance cost, can this series 
investigate:
1) A way to automatically fill the table from source code
2) A way to check if a patch is adding a new flow support but not documenting it

Also can you please propose a maintainer for it (can be documented in MAINTAINER 
file) who will be responsible of the correctness of the table, which means:
- Will verify a claimed support by a PMD is really supported
- All flow API features are documented for a PMD
- Changes in the code are reflected to the documentation
- Trace PMD maintainers for missing data