Re: [dpdk-dev] [PATCH v2] doc: add reserve fields to eventdev public structures

[Stephen Hemminger <stephen@networkplumber.org>]

On Wed, 5 Aug 2020 14:40:01 +0530
Jerin Jacob <jerinjacobk@gmail.com> wrote:

> On Wed, Aug 5, 2020 at 2:16 PM Kinsella, Ray <mdr@ashroe.eu> wrote:
> >
> >
> >
> > On 04/08/2020 17:20, Stephen Hemminger wrote:  
> > > On Tue, 4 Aug 2020 11:41:53 +0100
> > > Bruce Richardson <bruce.richardson@intel.com> wrote:
> > >  
> > >> On Mon, Aug 03, 2020 at 12:59:03PM +0530, pbhagavatula@marvell.com wrote:  
> > >>> From: Pavan Nikhilesh <pbhagavatula@marvell.com>
> > >>>
> > >>> Add 64 byte padding at the end of event device public structure to allow
> > >>> future extensions.
> > >>>
> > >>> Signed-off-by: Pavan Nikhilesh <pbhagavatula@marvell.com>
> > >>> Acked-by: Jerin Jacob <jerinj@marvell.com>
> > >>> ---
> > >>>  v2 Changes:
> > >>>  - Modify commit title.
> > >>>  - Add patch reference to doc.
> > >>>
> > >>>  doc/guides/rel_notes/deprecation.rst | 11 +++++++++++
> > >>>  1 file changed, 11 insertions(+)
> > >>>
> > >>> diff --git a/doc/guides/rel_notes/deprecation.rst b/doc/guides/rel_notes/deprecation.rst
> > >>> index ea4cfa7a4..ec5db68e9 100644
> > >>> --- a/doc/guides/rel_notes/deprecation.rst
> > >>> +++ b/doc/guides/rel_notes/deprecation.rst
> > >>> @@ -151,3 +151,14 @@ Deprecation Notices
> > >>>    Python 2 support will be completely removed in 20.11.
> > >>>    In 20.08, explicit deprecation warnings will be displayed when running
> > >>>    scripts with Python 2.
> > >>> +
> > >>> +* eventdev: A 64 byte padding is added at the end of the following structures
> > >>> +  in event device library to support future extensions:
> > >>> +  ``rte_event_crypto_adapter_conf``, ``rte_event_eth_rx_adapter_conf``,
> > >>> +  ``rte_event_eth_rx_adapter_queue_conf``, ``rte_event_eth_tx_adapter_conf``,
> > >>> +  ``rte_event_timer_adapter_conf``, ``rte_event_timer_adapter_info``,
> > >>> +  ``rte_event_dev_info``, ``rte_event_dev_config``, ``rte_event_queue_conf``,
> > >>> +  ``rte_event_port_conf``, ``rte_event_timer_adapter``,
> > >>> +  ``rte_event_timer_adapter_data``.
> > >>> +  Reference:
> > >>> +  http://patches.dpdk.org/project/dpdk/list/?series=10728&archive=both&state=*
> > >>> --  
> > >>
> > >> I don't like this idea of adding lots of padding to the ends of these
> > >> structures. For some structures, such as the public arrays for devices it
> > >> may be necessary, but for all the conf structures passed as parameters to
> > >> functions I think we can do better. Since these structures are passed by
> > >> the user to various functions, function versioning can be used to ensure
> > >> that the correct function in eventdev is always called. From there to the
> > >> individual PMDs, we can implement ABI compatibility by either:
> > >> 1. including the length of the struct as a parameter to the driver. (This is
> > >>   a bit similar to my proposal for rawdev [1])
> > >> 2. including the ABI version as a parameter to the driver.
> > >>
> > >> Regards
> > >> /Bruce
> > >>
> > >> [1] http://inbox.dpdk.org/dev/?q=enhance+rawdev+APIs  
> > >
> > > This is a bad idea.
> > >
> > > Reserved fields won't work because nothing requires that the application
> > > zero them. You can't start using them later because the application
> > > may put uninitialized or junk data there.
> > >  
> >
> > +1, to Stephens comments.  
> 
> Since the problem is not specific to one substem, if we need to add a
> field in config structures,
> What will the expected way of handling across the DPDK?

If you need fields go through the normal enhancement process, and get it
reviewed and put them in a major release milestone.
Sorry, there is no free lunch by adding reserved fields.

Look up YAGNI