Re: [dpdk-dev] [RFC 0/3] ethdev: add ptype as Rx offload

[Stephen Hemminger <stephen@networkplumber.org>]

On Wed, 7 Aug 2019 11:32:35 +0300
Andrew Rybchenko <arybchenko@solarflare.com> wrote:

> On 8/7/19 5:04 AM, Jerin Jacob Kollanukkaran wrote:
> >> -----Original Message-----
> >> From: Stephen Hemminger <stephen@networkplumber.org>
> >> Sent: Wednesday, August 7, 2019 4:45 AM
> >> To: Andrew Rybchenko <arybchenko@solarflare.com>
> >> Cc: Pavan Nikhilesh Bhagavatula <pbhagavatula@marvell.com>; Hemant
> >> Agrawal <hemant.agrawal@nxp.com>; Jerin Jacob Kollanukkaran
> >> <jerinj@marvell.com>; dev@dpdk.org
> >> Subject: [EXT] Re: [dpdk-dev] [RFC 0/3] ethdev: add ptype as Rx offload
> >>
> >> On Tue, 6 Aug 2019 12:06:35 +0300
> >> Andrew Rybchenko <arybchenko@solarflare.com> wrote:
> >>  
> >>> On 8/6/19 11:47 AM, Pavan Nikhilesh Bhagavatula wrote:  
> >>>>> -----Original Message-----
> >>>>> From: Hemant Agrawal <hemant.agrawal@nxp.com>
> >>>>> Sent: Tuesday, August 6, 2019 1:49 PM
> >>>>> To: Pavan Nikhilesh Bhagavatula <pbhagavatula@marvell.com>; Jerin
> >>>>> Jacob Kollanukkaran <jerinj@marvell.com>
> >>>>> Cc: dev@dpdk.org
> >>>>> Subject: RE: [dpdk-dev] [RFC 0/3] ethdev: add ptype as Rx offload  
> >>>>>> Add PTYPE to DEV_RX_OFFLOAD_* flags.
> >>>>>>
> >>>>>> Currently, most of the NICs already support PTYPE parsing and
> >>>>>> update  
> >>>>> the  
> >>>>>> mbuf->packet_type through an internal lookup table, but there is
> >>>>>> mbuf->no  
> >>>>> way to  
> >>>>>> disable the lookup if the application is not intrested in ptypes  
> >>>>> returned by  
> >>>>>> `rte_eth_dev_get_supported_ptypes`.
> >>>>>>  
> >>>>> [Hemant]  it will also mean introducing another check in datapath,
> >>>>> if the application has asked for PTYPE offload - copy the results
> >>>>> to mbuf-  
> >>>>>> packet_type otherwise don't do it.  
> >>>> I think that having the check would give better performance than
> >>>> loading ptype table to L1 doing  a lookup and copying it to mbuf when the  
> >> application doesn't need it.  
> >>> Anyway, if PMD decides that it is better to always provide packet type
> >>> information - there is no harm. Basically if the offload is not
> >>> requested it makes packet_type undefined in mbuf.
> >>>  
> >>>>> Your second patch is incomplete in the sense that it only adds the
> >>>>> capability. But it does not disable the lookups?  
> >>>> It is upto the maintainer of the PMD to disable the lookup in data
> >>>> path. If there is a scope of optimization then they could do it. There is no  
> >> harm in exposing  PTYPE even RX_OFFLOAD_PTYPE is not enabled.  
> >>>> I was hesitant to touch data path as it would be impossible to verify  
> >> performance effect on all NICs.  
> >>> I think it is the right way to approach it especially taking
> >>> transition into account.
> >>>  
> >> With hardline API policy, this has to fail on compile for old applications.  
> 
> Stephen, could you explain a bit more why.

Existing releases packets will be received with ptype for hardware that
supports it. We should not require users to change their application to
continue to get mbufs with ptype.  If your change would break that, and
require application to change; then your change should break the API in
a hard way that causes compile rather than runtime failure.

The best solution would be to just keep old applications running and compiling
without breaking anything. That means ptype should still be received.

If (as an optimization) you want to allow application to turn of getting
ptype; then that would be a useful. Probably best done at the port level
as part of configuration.

> 
> > Not specific to this API change. That's is the propriety any new symbol addition
> > to the code base.
> >
> > Planning to make this API change available fromv19.11 LTS.  
> 
> The only way to to require applications to enable PTYPE offload to get
> ptypes in mbuf since 19.11 LTS is to have deprecation notice in 19.08.
> 
> >> You can't magically assume that applications using ptype will set new feature.  
> > When OFFLOAD flags got introduced, we decided to disable all offloads by default.
> > So, need to add positive logic here to enable offload instead of enable something by
> > Default and disable if required to get have synergy with other offloads.
> >
> > Will update the release note as usual to document the change.
> > Since there is NO ABI change, IMO, we don't need deprecation notice.  
> 
> Sorry, but it is a behaviour change. Before an application does not need
> to enable ptype offload, but now it is required. It means that application
> will be broken and, therefore, it requires deprecation notice.

The DPDK development community has to make not breaking applications
a higher priority than adding marginal enhancements