Re: [dpdk-dev] [PATCH] mbuf: document rule for new fields and flags

[Jerin Jacob <jerinjacobk@gmail.com>]

On Tue, May 26, 2020 at 9:36 PM Olivier Matz <olivier.matz@6wind.com> wrote:
>
> Hi,
>
> On Tue, May 26, 2020 at 01:09:37PM +0530, Jerin Jacob wrote:
> > On Tue, May 26, 2020 at 2:54 AM Thomas Monjalon <thomas@monjalon.net> wrote:
> > >
> > > Since dynamic fields and flags were added in 19.11,
> > > the idea was to use them for new features, not only PMD-specific.
> > >
> > > The rule is made more explicit in doxygen, in the mbuf guide,
> > > and in the contribution design guidelines.
> > >
> > > For more information about the original design, see the presentation
> > > https://www.dpdk.org/wp-content/uploads/sites/35/2019/10/DynamicMbuf.pdf
> > >
> > > Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
> > > ---
> > >  doc/guides/contributing/design.rst | 13 +++++++++++++
> > >  doc/guides/prog_guide/mbuf_lib.rst | 23 +++++++++++++++++++++++
> > >  lib/librte_mbuf/rte_mbuf_core.h    |  2 ++
> > >  3 files changed, 38 insertions(+)
> > >
> > > diff --git a/doc/guides/contributing/design.rst b/doc/guides/contributing/design.rst
> > > index d3dd694b65..508115d5bd 100644
> > > --- a/doc/guides/contributing/design.rst
> > > +++ b/doc/guides/contributing/design.rst
> > > @@ -57,6 +57,19 @@ The following config options can be used:
> > >  * ``CONFIG_RTE_EXEC_ENV`` is a string that contains the name of the executive environment.
> > >  * ``CONFIG_RTE_EXEC_ENV_FREEBSD`` or ``CONFIG_RTE_EXEC_ENV_LINUX`` are defined only if we are building for this execution environment.
> > >
> > > +Mbuf features
> > > +-------------
> > > +
> > > +The ``rte_mbuf`` structure must be kept small (128 bytes).
> > > +
> > > +In order to add new features without wasting buffer space for unused features,
> > > +some fields and flags can be registered dynamically in a shared area.
> >
> > I think, instead of "can", it should be "must"
> >
> > > +The "dynamic" mbuf area is the default choice for the new features.
>
> In my opinion, Thomas' proposal is correct, with the next sentence
> saying it is the default choice for new features.
>
> Giving guidelines is a good thing (thanks Thomas for documenting it),
> but I don't think we should be too strict: the door remains open for
> technical debate and exceptions.

If you are open for the exception then it must be mention in what case
the exception is allowed and what are the criteria of the exception?

For example,  Why did n't we choose the following patch as expectation
http://patches.dpdk.org/patch/68733/  even if only one bit used.

If we are not not defining the criteria, IMO, This patch serve no purpose than
the existing situation.

Do you think, any case where the dynamic scheme can not be used as a replacement
for static other than performance hit.