[dpdk-dev] [RFC] ethdev: reserve the RX offload most-significant bits for PMD scartch

[Andrew Rybchenko]

On 6/21/19 4:12 AM, Wang, Haiyue wrote:
>
> Not so frightening in real world for an application to be aware of its 
> NICs:
>
> https://github.com/Juniper/contrail-vrouter/blob/master/dpdk/vr_dpdk_ethdev.c#L387 
> <https://urldefense.proofpoint.com/v2/url?u=https-3A__github.com_Juniper_contrail-2Dvrouter_blob_master_dpdk_vr-5Fdpdk-5Fethdev.c-23L387&d=DwMGaQ&c=euGZstcaTDllvimEN8b7jXrwqOf-v5A_CdpgnVfiiMM&r=flTOx6Av679My7o_iScb5sOlLD68bpUyE2RUtfW3SWQ&m=XSIm84nALkE7O1aeqpJkVJJWzepVsGEJsTeiDCxoLK4&s=L1vEJ5GeVHbammKc0iJn0YdoeKf0GqeeNJy-q5xCi6E&e=>
>
>

In this particular case it is just a workaround for bonding and bnxt.
Driver name is provided and sufficient to make it possible when
absolutely required.

> Yes, we need to avoid this kind of design.
>
> BR,
>
> Haiyue
>
> *From:*Andrew Rybchenko [mailto:arybchenko at solarflare.com]
> *Sent:* Friday, June 21, 2019 02:30
> *To:* Wang, Haiyue <haiyue.wang at intel.com>; dev at dpdk.org
> *Cc:* Yigit, Ferruh <ferruh.yigit at intel.com>; Thomas Monjalon 
> <thomas at monjalon.net>
> *Subject:* Re: [dpdk-dev] [RFC] ethdev: reserve the RX offload 
> most-significant bits for PMD scartch
>
> CC ethdev maintainers
>
> On 6/20/19 10:25 AM, Haiyue Wang wrote:
>
>     Generally speaking, the DEV_RX_OFFLOAD_xxx for RX offload capabilities
>
>     of a device is one-bit field definition, it has 64 different values at
>
>     most.
>
>     Nowdays the receiving queue of NIC has rich features not just checksum
>
>     offload, like it can extract the network protocol header fields to its
>
>     RX descriptors for quickly handling. But this kind of feature is not so
>
>     common, and it is hardware related. Normally, this can be done through
>
>     rte_devargs driver parameters, but the scope is per device. This is not
>
>     so nice for per queue design.
>
>     The per queue API 'rte_eth_rx_queue_setup' and data structure 'struct
>
>     rte_eth_rxconf' are stable now, and be common for all PMDs. For keeping
>
>     the ethdev API & ABI compatibility, and the application can make good
>
>     use of the NIC's specific features, reserving the most-significant bits
>
>     of RX offload seems an compromise method.
>
>     Then the PMDs redefine these bits as they want, all PMDs share the same
>
>     bit positions and expose their new definitions with the header file.
>
>     The experimental reserved bits number is 6 currently. Tt can be one-bit
>
>     for each features up to the the maximum number 6. It can also be some
>
>     bits encoding: e.g, 6 bits can stand for 63 maximum number of features.
>
>     We call these reserved bits as DEV_RX_OFFLOAD_PMD_SCRATCH. And the left
>
>     unused bits number is : 64 - 19 (currently defined) - 6 (PMD scartch) =
>
>     39.
>
>     This is not so nice for applications, they need to check PMD's driver
>
>     name for lookuping their DEV_RX_OFFLOAD_PMD_SCRATCH definitions. But it
>
>     is good for the applications to make use of the hardware compatibility.
>
>     Signed-off-by: Haiyue Wang<haiyue.wang at intel.com>  <mailto:haiyue.wang at intel.com>
>
>
> I would say that it very bad for applications. It sounds like reserved 
> bits
> in registers which have meaning in fact and sometimes different meaning.
> Of course, it is not that bad when rules are defined, but such kind of
> features tend to spread and clutter up interfaces. IMHO, the feature is
> really frightening.
>