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

[Stephen Hemminger <stephen@networkplumber.org>]

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 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.
You can't magically assume that applications using ptype will set new feature.