Re: [dpdk-dev] [PATCH] ethdev: fix queue mapping documentation

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

On 7/10/2018 8:06 AM, Andrew Rybchenko wrote:
> On 10.07.2018 09:20, Jerin Jacob wrote:
>> -----Original Message-----
>>> Date: Mon, 2 Jul 2018 16:45:33 +0100
>>> From: Ferruh Yigit <ferruh.yigit@intel.com>
>>> To: Andrew Rybchenko <arybchenko@solarflare.com>, Jerin Jacob
>>>   <jerin.jacob@caviumnetworks.com>, dev@dpdk.org
>>> CC: thomas@monjalon.net, stable@dpdk.org
>>> Subject: Re: [dpdk-dev] [PATCH] ethdev: fix queue mapping documentation
>>> User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101
>>>   Thunderbird/52.8.0
>>>
>>>
>>> On 7/2/2018 4:32 PM, Andrew Rybchenko wrote:
>>>> On 07/02/2018 06:08 PM, Ferruh Yigit wrote:
>>>>> On 6/29/2018 10:44 AM, Jerin Jacob wrote:
>>>>>> The RTE_MAX_ETHPORT_QUEUE_STATS_MAPS does not exists, change
>>>>>> to the correct definition(RTE_ETHDEV_QUEUE_STAT_CNTRS)
>>>>>>
>>>>>> Fixes: 5de201df8927 ("ethdev: add stats per queue")
>>>>>> Cc: stable@dpdk.org
>>>>>>
>>>>>> Signed-off-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>
>>>>>> ---
>>>>>>   lib/librte_ethdev/rte_ethdev.h | 4 ++--
>>>>>>   1 file changed, 2 insertions(+), 2 deletions(-)
>>>>>>
>>>>>> diff --git a/lib/librte_ethdev/rte_ethdev.h b/lib/librte_ethdev/rte_ethdev.h
>>>>>> index 36e3984ea..375ea24ce 100644
>>>>>> --- a/lib/librte_ethdev/rte_ethdev.h
>>>>>> +++ b/lib/librte_ethdev/rte_ethdev.h
>>>>>> @@ -2144,7 +2144,7 @@ void rte_eth_xstats_reset(uint16_t port_id);
>>>>>>    * @param stat_idx
>>>>>>    *   The per-queue packet statistics functionality number that the transmit
>>>>>>    *   queue is to be assigned.
>>>>>> - *   The value must be in the range [0, RTE_MAX_ETHPORT_QUEUE_STATS_MAPS - 1].
>>>>>> + *   The value must be in the range [0, RTE_ETHDEV_QUEUE_STAT_CNTRS - 1].
>>>>> Yes RTE_MAX_ETHPORT_QUEUE_STATS_MAPS doesn't exits and comment is wrong, but
>>>>> RTE_ETHDEV_QUEUE_STAT_CNTRS also slightly not correct.
>>>>>
>>>>> I think how testpmd uses it increase the confusion.
>>>>>
>>>>> In ixgbe there is no stats registers per queue, 128 queues are represented by 16
>>>>> register set. stat_idx here is the index of that 16 registers. You map queue to
>>>>> stats requester to get queue stats.
>>>>>
>>>>> Also there is RTE_ETHDEV_QUEUE_STAT_CNTRS config in the ethdev API, which is the
>>>>> hardcoded size of the queue stats, its default value is 16. This limits number
>>>>> of the queues we can get stats from but saves allocated space. (Why not dynamic?)
>>>>>
>>>>> You can increase the RTE_ETHDEV_QUEUE_STAT_CNTRS to the max supported number of
>>>>> queue and ethdev code will be all valid. But "stat_idx" can't go beyond 16 (for
>>>>> ixgbe) because it is hardware limitation and it may change from hw to hw.
>>>>>
>>>>> Also technically it should be possible to reduce RTE_ETHDEV_QUEUE_STAT_CNTRS to
>>>>> a low number, like 2, but in ixgbe map two queues into stat registers 14 & 15
>>>>> and display those two set as queue stat 0 and 1. It seems current implementation
>>>>> prevents this and forces the queues mapped should be less than
>>>>> RTE_ETHDEV_QUEUE_STAT_CNTRS. Overall it seems there is a mixed used of
>>>>> RTE_ETHDEV_QUEUE_STAT_CNTRS and stats queue index values, I assume because both
>>>>> are same values.
>>>>>
>>>>> I suggest updating it as:
>>>>> "
>>>>> The value must be in the range:
>>>>>   [0 - MIN(HW Stat Registers Size, RTE_ETHDEV_QUEUE_STAT_CNTRS) - 1]
>>>>> "
>>>> Technically I think it is not a problem to specify more than HW supports.
>>>> The function should simply return error.  RTE_ETHDEV_QUEUE_STAT_CNTRS is
>>>> a hard limit which should be checked by ethdev.
>>>> The reasonable next question is how to find out what is the maximum for PMD/HW.
>>>> I think it deserves entry in dev_info. May be not now.
>>> Yes there is not a way to find out that limit by application, setting
>>> RTE_ETHDEV_QUEUE_STAT_CNTRS to 16 and using it as limit solving the issue for now :)
>>
>> If I understand it correctly, in the documentation, we are specify the
>> limits to avoid the segfault etc and if the specific PMD does not
>> support the range up to RTE_ETHDEV_QUEUE_STAT_CNTRS, It can simply return
>> error which makes the documentation semantically correct.
>>
>> Considering the above point, I think this patch is correct considering
>> there is no way currently to detect the limit supported by PMD. So,
>> 1) Should we keep the patch as is?
>>
>> --
>> The value must be in the range [0, RTE_ETHDEV_QUEUE_STAT_CNTRS - 1].
>> --
>>
>> OR
>>
>> 2) Change to
>>
>> --
>> The value must be in the range:
>> [0 - MIN(Device max per Tx queue stats, RTE_ETHDEV_QUEUE_STAT_CNTRS) - 1]
>> --
>>
>> I prefer option 1. But I am okay send v2 if any ethdev maintainers prefer
>> option 2.
>>
>> Let me know.
> 
> I would prefer option 1 as well since right now we have no way to
> advertise/find out "Device max per Tx queue stats".

OK, agreed/convinced on option 1, eventually it will be better than current
comment, so no new version required, thanks.