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

[Stephen Hemminger]

On Tue, 4 Aug 2020 11:41:53 +0100
Bruce Richardson <bruce.richardson at intel.com> wrote:

> On Mon, Aug 03, 2020 at 12:59:03PM +0530, pbhagavatula at marvell.com wrote:
> > From: Pavan Nikhilesh <pbhagavatula at marvell.com>
> > 
> > Add 64 byte padding at the end of event device public structure to allow
> > future extensions.
> > 
> > Signed-off-by: Pavan Nikhilesh <pbhagavatula at marvell.com>
> > Acked-by: Jerin Jacob <jerinj at 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.