Re: [dpdk-dev] [PATCH v2] ethdev: document RSS default key and types

[Shahaf Shuler <shahafs@mellanox.com>]

Hi Adrien, 

Tuesday, November 13, 2018 7:15 PM, Adrien Mazarguil:
> Subject: Re: [dpdk-dev] [PATCH v2] ethdev: document RSS default key and
> types
> 
> Again a bit late to the party, please see below.
> 
> On Sun, Nov 11, 2018 at 09:35:22AM +0000, Ori Kam wrote:

[...]

> > > The setfault is the result of commit a4391f8bae ("app/testpmd: set
> > > default RSS key as null").
> > > Reverting this commit should fix the segfault but it also means
> > > there is no way to set default key (key=NULL) with testpmd.
> > > Need to check if this is only a testpmd limitation and not all
> > > applications limitation.
> > >
> > > We should decide how an application can set default RSS without
> > > knowing anything about keys.
> > >
> >
> > I agree with Adrian that the main criteria should be the length.
> > Maybe the set default RSS in testpmd should get new parameter.
> 
> Since [1] was reverted and we seem to agree that a zero key_len should
> trigger a PMD-specific default key, this can already be requested with
> testpmd by overriding key_len, e.g.:
> 
>  flow create 1 pattern eth / end actions rss key_len 0 / end
> 
> Using an empty string as the key would yield the same result but cannot be
> expressed on the command line yet. Note that specifying a key automatically
> overrides key_len, so key_len must be forced to 0 last to get PMD defaults:
> 
>  flow create 1 pattern eth / end actions rss key foo key_len 0 / end

I don't understand why we are backing up API claims with "how testpmd is implemented". The APIs should be correct, regardless of how testpmd is using them. 

To this doc issue, 
I don't understand on what cases it makes sense for application to have rss_key_len = 0 while rss_key != NULL. This is obviously in-consist input, and of course all PMD will just ignore the key.
I think enforcing rss_key and rss_key_len to be NULL is a fair requirement from application, and it makes no confusion in the API inputs.

> 
> Here key_len is set to testpmd's default size when parsing "rss", updated to
> 3 when parsing "key foo" and updated once again when parsing "key_len 0".
> 
> Lastly, while it would make sense for testpmd to use 0 as the default value,
> doing so yields inconsistent balancing results between vendors/devices as
> they all come with a different key. Same reason as initializing the RSS types
> field to the global rss_hf instead of 0.



> 
> [1] "app/testpmd: revert setting default RSS"
> 
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> 
> --
> Adrien Mazarguil
> 6WIND