Re: [dpdk-dev] [PATCH] doc: announce changes to ethdev rxconf structure

[Andrew Rybchenko <arybchenko@solarflare.com>]

Hi Stephen,

On 8/30/20 9:26 PM, Stephen Hemminger wrote:
> On Sun, 30 Aug 2020 15:58:57 +0300
> Andrew Rybchenko <arybchenko@solarflare.com> wrote:
> 
>>>>>>>
>>>>>>> The non-zero value of rx_split_num field configures the receiving
>>>>>>> queue to split ingress packets into multiple segments to the mbufs
>>>>>>> allocated from various memory pools according to the specified
>>>>>>> lengths. The zero value of rx_split_num field provides the backward
>>>>>>> compatibility and queue should be configured in a regular way (with
>>>>>>> single/multiple mbufs of the same data buffer length allocated from
>>>>>>> the single memory pool).  
>>>>>>
>>>>>> From the above description it is not 100% clear how it will coexist with:
>>>>>>  - existing mb_pool argument of the rte_eth_rx_queue_setup()
>>>>>>  - DEV_RX_OFFLOAD_SCATTER  
>>>>>
>>>>> DEV_RX_OFFLOAD_SCATTER flag is required to be reported and configured
>>>>> for the new feature to indicate the application is prepared for the
>>>>> multisegment packets.  
>>>>
>>>> I hope it will be mentioned in the feature documentation in the future, but
>>>> I'm not 100% sure that it is required. See below.  
>>> I suppose there is the hierarchy:
>>> - applications configures DEV_RX_OFFLOAD_SCATTER on the port and tells in this way:
>>> "Hey, driver, I'm ready to handle multi-segment packets". Readiness in general.
>>> - application configures BUFFER_SPLIT and tells PMD _HOW_ it wants to split, in particular way:
>>> "Hey, driver, please, drop ten bytes here, here and here, and the rest - over there"  
>>
>> My idea is to keep SCATTER and BUFFER_SPLIT independent.
>> SCATTER is a possibility to make multi-segment packets getting
>> mbufs from main rxq mempool as many as required.
>> BUFFER_SPLIT is support of many mempools and splitting
>> received packets as specified.
> 
> No.
> Once again, drivers should take anything from application and rely on using
> logic to choose best path. Modern CPU's have good branch predictors, and making
> the developer do that work is counter productive.

Please, add a bit more details. I simply can see relationship.
So, right now for me it looks like just misunderstanding.

Thanks,
Andrew.