Re: [dpdk-dev] [PATCH] net/ring: advertise multi segment support.

[Ferruh Yigit <ferruh.yigit@intel.com>]

On 9/28/2020 12:01 PM, Bruce Richardson wrote:
> On Mon, Sep 28, 2020 at 11:25:34AM +0100, Ferruh Yigit wrote:
>> On 9/28/2020 8:31 AM, Dumitru Ceara wrote:
>>> On 9/22/20 4:21 PM, Ferruh Yigit wrote:
>>>> On 9/18/2020 11:36 AM, Dumitru Ceara wrote:
>>>>> Even though ring interfaces don't support any other TX/RX offloads they
>>>>> do support sending multi segment packets and this should be advertised
>>>>> in order to not break applications that use ring interfaces.
>>>>>
>>>>
>>>> Does ring PMD support sending multi segmented packets?
>>>>
>>>
>>> Yes, sending multi segmented packets works fine with ring PMD.
>>>
>>
>> Define "works fine" :)
>>
>> All PMDs can put the first mbuf of the chained mbuf to the ring, in that
>> case what is the difference between the ones supports
>> 'DEV_TX_OFFLOAD_MULTI_SEGS' and the ones doesn't support?
>>
>> If the traffic is only from ring PMD to ring PMD, you won't recognize the
>> difference between segmented or not-segmented mbufs, and it will look like
>> segmented packets works fine.
>> But if there is other PMDs involved in the forwarding, or if need to process
>> the packets, will it still work fine?
>>
> 
> What other PMDs do or don't do should be irrelevant here, I think. The fact
> that multi-segment PMDs make it though the ring PMD in valid form should be
> sufficient to mark it as supported.
> 
>>>> As far as I can see ring PMD doesn't know about the mbuf segments.
>>>>
>>>
>>> Right, the PMD doesn't care about the mbuf segments but it implicitly
>>> supports sending multi segmented packets. From what I see it's actually
>>> the case for most of the PMDs, in the sense that most don't even check
>>> the DEV_TX_OFFLOAD_MULTI_SEGS flag and if the application sends multi
>>> segment packets they are just accepted.
>>>
>>
>> As far as I can see, if the segmented packets sent, the ring PMD will put
>> the first mbuf into the ring without doing anything specific to the next
>> segments.
>>
>> If the 'DEV_TX_OFFLOAD_MULTI_SEGS' is supported I expect it should detect
>> the segmented packets and put each chained mbuf into the separate field in
>> the ring.
>>
> 
> Why, what would be the advantage of that? Right now if you send in a valid
> packet chain to the Ring PMD, you get a valid packet chain out again the
> other side, so I don't see what needs to change about that behaviour.
> 

Got it. Konstantin also had similar comment, I have replied there.

>>>
>>> However, the fact that the ring PMD doesn't advertise this implicit
>>> support forces applications that use ring PMD to have a special case for
>>> handling ring interfaces. If the ring PMD would advertise
>>> DEV_TX_OFFLOAD_MULTI_SEGS this would allow upper layers to be oblivious
>>> to the type of underlying interface.
>>>
>>
>> This is not handling the special case for the ring PMD, this is why he have
>> the offload capability flag. Application should behave according capability
>> flags, not per specific PMD.
>>
>> Is there any specific usecase you are trying to cover?