Re: [PATCH 3/3] net/mlx5: fix missing adjustment MPRQ stride devargs

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

On 12/9/2021 12:33 PM, Kevin Traynor wrote:
> On 08/12/2021 15:40, Matan Azrad wrote:
>> Hi Ferruh
>>
>> Thanks for the review.
>>
>> Please see inside some clarifications.
>>
>> From: Ferruh Yigit
>>> On 12/8/2021 12:52 PM, Michael Baum wrote:
>>>>
>>>> On 12/07/2021 3:41 PM, ferruh.yigit@intel.com wrote:
>>>>>
>>>>> On 11/23/2021 6:38 PM, michaelba@nvidia.com wrote:
>>>>>> From: Michael Baum<michaelba@nvidia.com>
>>>>>>
>>>>>> In Multy-Packet RQ creation, the user can choose the number of
>>>>>> strides
>>>>>
>>>>> Multi-Packet ?
>>>>
>>>> Yes, you're right. It should have been Multi-Packet, thank you for that.
>>>>
>>>>>
>>>>>> and their size in bytes. The user updates it using specific devargs
>>>>>> for both of these parameters.
>>>>>> The above two parameters determine the size of the WQE which is
>>>>>> actually their product of multiplication.
>>>>>>
>>>>>> If the user selects values that are not in the supported range, the
>>>>>> PMD changes them to default values. However, apart from the range
>>>>>> limitations for each parameter individually there is also a minimum
>>>>>> value on their multiplication. When the user selects values that
>>>>>> their multiplication are lower than minimum value, no adjustment is
>>>>>> made and the creation of the WQE fails.
>>>>>>> This patch adds an adjustment in these cases as well. When the user
>>>>>> selects values whose multiplication is lower than the minimum, they
>>>>>> are replaced with the default values.
>>>>>>
>>>>>> Fixes: ecb160456aed ("net/mlx5: add device parameter for MPRQ stride
>>>>>> size") Cc:stable@dpdk.org
>>>>>>
>>>>>
>>>>> Again, not sure if we can backport this patch, this looks a behavior
>>>>> change more than a fix.
>>>>>
>>>>> Previously if the user provided values ends up being invalid, PMD
>>>>> seems returning error.
>>>>> With this patch, instead of returning error PMD prefers to use
>>>>> default values and doesn't return error.
>>>>
>>>> It isn't behavior change.
>>>> It existed before, except that it is concentrated into one function.
>>>>
>>>>>
>>>>> I am not sure if it is correct thing to ignore (adjust) user provided
>>>>> values, but that can be up to the PMD as long as the behavior is
>>> documented.
>>>>
>>>> Adjustment is the likely thing to do because the range depends on the
>>> device and the user does not necessarily know it.
>>>> This behavior is documented here
>>>> https://doc.dpdk.org/guides/nics/mlx5.html#run-time-configuration
>>>> (Run-time configuration -> Driver options -> mprq_log_stride_num/size)
>>>>
>>>
>>> It is documented that adjustments will be done if any specific argument is
>>> not in the range of the device capability.
>>>
>>> It is not clear what will happen if the calculated value from both variables are
>>> not valid.
>>
>> The driver should adjust it to a legal value.
>>
>>> If it is not documented before, and previously it was returning error, now
>>> adjusting values to make it work looks like behavior change to me.
>>
>> The driver should not return an error - the driver should adjust to a legal value in case of illegal values by the user.
>> It is documented in the devargs description.
>>
>> Not behavior change but a bug fix; previously, the adjustment may return an error(which is a bug) or cause unexpected behavior in the HW(which is an old FW bug).
>> Now, no error, no unexpected behavior - bug should be fixed for any FW version.
>>
> 
> I can understand both arguments. It is a behaviour change as the user will see a different behaviour for a given set of values.
> 
> However, each parameter is already validated and defaults are provided as backup. The combination not being checked could be seen a piece of missed validation for those values and a bug. In this case, given it is unlikely any user would be happy with the WQE creation failure, i think it is ok to backport the missing validation/adjustment.
> 
>>> This is more of a process question, than technical detail in the driver, so
>>> @Luca, @Kevin, @Christian, can you please comment? I will follow your
>>> suggestion.
>>>
> 
> Thanks for raising it Ferruh.
> 


Proceeding with patch then, updated fixes tag for 2/3 in next-net.