Re: [dpdk-dev] [PATCH v6 1/6] ethdev: introduce Rx buffer split

[Jerin Jacob <jerinjacobk@gmail.com>]

On Thu, Oct 15, 2020 at 1:13 PM Slava Ovsiienko <viacheslavo@nvidia.com> wrote:
>
> Hi, Jerin

Hi Slava,

>
> > -----Original Message-----
> > From: Jerin Jacob <jerinjacobk@gmail.com>
> > Sent: Wednesday, October 14, 2020 21:57
> > To: Slava Ovsiienko <viacheslavo@nvidia.com>
> > Cc: dpdk-dev <dev@dpdk.org>; NBU-Contact-Thomas Monjalon
> > <thomas@monjalon.net>; Stephen Hemminger
> > <stephen@networkplumber.org>; Ferruh Yigit <ferruh.yigit@intel.com>;
> > Olivier Matz <olivier.matz@6wind.com>; Maxime Coquelin
> > <maxime.coquelin@redhat.com>; David Marchand
> > <david.marchand@redhat.com>; Andrew Rybchenko
> > <arybchenko@solarflare.com>
> > Subject: Re: [PATCH v6 1/6] ethdev: introduce Rx buffer split
> >
> > On Wed, Oct 14, 2020 at 11:42 PM Viacheslav Ovsiienko
> > <viacheslavo@nvidia.com> wrote:
> > >
> > > The DPDK datapath in the transmit direction is very flexible.
> > > An application can build the multi-segment packet and manages almost
> > > all data aspects - the memory pools where segments are allocated from,
> > > the segment lengths, the memory attributes like external buffers,
> > > registered for DMA, etc.
> > >
>
> [..snip..]
>
> > > For example, let's suppose we configured the Rx queue with the
> > > following segments:
> > >     seg0 - pool0, len0=14B, off0=2
> > >     seg1 - pool1, len1=20B, off1=128B
> > >     seg2 - pool2, len2=20B, off2=0B
> > >     seg3 - pool3, len3=512B, off3=0B
> >
> >
> > Sorry for chime in late. This API lookout looks good to me.
> > But, I am wondering how the application can know the capability or "limits" of
> > struct rte_eth_rxseg structure for the specific PMD. The other descriptor limit,
> > it's being exposed with struct rte_eth_dev_info::rx_desc_lim; If PMD can
> > support a specific pattern rather than returning the blanket error, the
> > application should know the limit.
> > IMO, it is better to add
> > struct rte_eth_rxseg *rxsegs;
> > unint16_t nb_max_rxsegs
> > in rte_eth_dev_info structure to express the capablity.
> > Where the en and offset can define the max offset.
> >
> > Thoughts?
>
> Moreover, there might be implied a lot of various limitations - offsets might be not supported at all or
> have some requirements for alignment, the similar requirements might be applied to segment size
> (say, ask for some granularity). Currently it is not obvious how to report all nuances, and it is supposed
> the limitations of this kind must be documented in PMD chapter. As for mlx5 - it has no special
> limitations besides common requirements to the regular segments.

Reporting the limitation in the documentation will not help for the
generic applications.

>
> One more point - the split feature might be considered as just one of possible cases of using
> these segment descriptions, other features might impose other (unknown for now) limitations.
> If we see some of the features of such kind or other PMDs adopts the split feature - we'll try to find
> the common root and consider the way how to report it.

My only concern with that approach will be ABI break again if
something needs to exposed over rte_eth_dev_info().
IMO, if we featured needs to completed only when its capabilities are
exposed in a programmatic manner.
As of mlx5, if there not limitation then info
rte_eth_dev_info::rxsegs[x].len, offset etc as UINT16_MAX so
that application is aware of the state.

>
> With best regards, Slava
>