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

[Andrew Rybchenko]

On 10/15/20 10:43 AM, Slava Ovsiienko wrote:
> Hi, Jerin
> 
>> -----Original Message-----
>> From: Jerin Jacob <jerinjacobk at gmail.com>
>> Sent: Wednesday, October 14, 2020 21:57
>> To: Slava Ovsiienko <viacheslavo at nvidia.com>
>> Cc: dpdk-dev <dev at dpdk.org>; NBU-Contact-Thomas Monjalon
>> <thomas at monjalon.net>; Stephen Hemminger
>> <stephen at networkplumber.org>; Ferruh Yigit <ferruh.yigit at intel.com>;
>> Olivier Matz <olivier.matz at 6wind.com>; Maxime Coquelin
>> <maxime.coquelin at redhat.com>; David Marchand
>> <david.marchand at redhat.com>; Andrew Rybchenko
>> <arybchenko at 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 at 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.
> 
> 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.

At least there are few simple limitations which are easy to
express:
 1. Maximum number of segments
 2. Possibility to use the last segment many times if required
    (I was suggesting to use scatter for it, but you rejected
     the idea - may be time to reconsider :) )
 3. Maximum offset
    Frankly speaking I'm not sure why it cannot be handled on
    PMD level (i.e. provide descriptors with offset taken into
    account or guarantee that HW mempool objects initialized
    correctly with required headroom). May be in some corner
    cases when the same HW mempool is shared by various
    segments with different offset requirements.
 4. Offset alignment
 5. Maximum/minimum length of a segment
 6. Length alignment

I realize that 3, 4 and 5 could be per segment number.
If it is really that complex, report common denominator
which is guaranteed to work. If we have no checks on ethdev
layer, application can ignore it if it knows better.