Re: [dpdk-dev] [PATCH v3 1/4] ethdev: add a field for rxq info structure

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

On 9/2/2020 8:19 AM, Matan Azrad wrote:
> 
> Hi Chengchang
> 
> From: Chengchang Tang
>> Hi, Matan
>>
>> On 2020/9/1 23:33, Matan Azrad wrote:
>>>
>>> Hi Chengchang
>>>
>>> Please see some question below.
>>>
>>> From: Chengchang Tang
>>>> Add a field named rx_buf_size in rte_eth_rxq_info to indicate the
>>>> buffer size used in receiving packets for HW.
>>>>
>>>> In this way, upper-layer users can get this information by calling
>>>> rte_eth_rx_queue_info_get.
>>>>
>>>> Signed-off-by: Chengchang Tang <tangchengchang@huawei.com>
>>>> Reviewed-by: Wei Hu (Xavier) <xavier.huwei@huawei.com>
>>>> Acked-by: Andrew Rybchenko <arybchenko@solarflare.com>
>>>> ---
>>>>  lib/librte_ethdev/rte_ethdev.h | 2 ++
>>>>  1 file changed, 2 insertions(+)
>>>>
>>>> diff --git a/lib/librte_ethdev/rte_ethdev.h
>>>> b/lib/librte_ethdev/rte_ethdev.h index 70295d7..9fed5cb 100644
>>>> --- a/lib/librte_ethdev/rte_ethdev.h
>>>> +++ b/lib/librte_ethdev/rte_ethdev.h
>>>> @@ -1420,6 +1420,8 @@ struct rte_eth_rxq_info {
>>>>         struct rte_eth_rxconf conf; /**< queue config parameters. */
>>>>         uint8_t scattered_rx;       /**< scattered packets RX supported. */
>>>>         uint16_t nb_desc;           /**< configured number of RXDs. */
>>>> +       /**< buffer size used for hardware when receive packets. */
>>>> +       uint16_t rx_buf_size;
>>>
>>> Is it the maximum supported Rx buffer by the HW?
>>> If yes, maybe max_rx_buf_size is better name?
>>
>> No, it is the Rx buffer size currently used by HW.
> 
> Doesn't it defined by the user? Using Rx queue mem-pool mbuf room size?
> 
> And it may be different per Rx queue....

There is no explicit configuration for Rx buffer size, PMDs does as you said
above and set mbuf data size as Rx buffer size, but this is not a defined rule,
technically PMD is free to select any size smaller than mbuf data size as their
Rx buffer size.
And this new field is to feed this configured Rx buffer size information back to
application.

> 
>> IMHO, the structure rte_eth_rxq_info and associated query API are mainly
>> used to query HW configurations at runtime or after queue is
>> configured/setup. Therefore, the content of this structure should be the
>> current HW configuration.
> 
> It looks me more like capabilities...
> The one which define the current configuration is the user by the configuration APIs(after reading the capabilities).
> 
> I don't think we have here all the current configurations, so what is special in this one?
> 
> 
>>> Maybe document that 0 means - no limitation by HW?
>>
>> Yes, there is no need to fill this filed for HW that has no restrictions on it.
>> I'll add it in v4.
>>
>>> Must application read it in order to know if its datapath should handle
>> multi-segment buffers?
>>
>> I think it's more appropriate to use scattered_rx to determine if multi-
>> segment buffers should be handled.
>>
>>>
>>> Maybe it will be good to force application to configure scatter when this
>> field is valid and smaller than max_rx_pkt_len\max_lro.. (<= room size)...
> 
> Can you explain more what is the issue you came to solve?
> 
>>>
>>>>  } __rte_cache_min_aligned;
>>>>
>>>>  /**
>>
>