Re: [dpdk-dev] [dpdk-stable] [PATCH v5 2/2] ethdev: fix docs of drivers callbacks getting xstats by IDs

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

On 9/30/2021 3:01 PM, Andrew Rybchenko wrote:
> On 9/30/21 3:08 PM, Ferruh Yigit wrote:
>> On 9/29/2021 12:54 PM, Andrew Rybchenko wrote:
>>> On 9/29/21 11:44 AM, Ferruh Yigit wrote:
>>>> On 9/28/2021 5:53 PM, Andrew Rybchenko wrote:
>>>>> On 9/28/21 7:50 PM, Ferruh Yigit wrote:
>>>>>> On 9/28/2021 1:05 PM, Andrew Rybchenko wrote:
>>>>>>> From: Ivan Ilchenko <ivan.ilchenko@oktetlabs.ru>
>>>>>>>
>>>>>>> Update xstats by IDs callbacks documentation in accordance with
>>>>>>> ethdev usage of these callbacks. Document valid combinations of
>>>>>>> input arguments to make driver implementation simpler.
>>>>>>>
>>>>>>> Fixes: 79c913a42f0 ("ethdev: retrieve xstats by ID")
>>>>>>> Cc: stable@dpdk.org
>>>>>>>
>>>>>>> Signed-off-by: Ivan Ilchenko <ivan.ilchenko@oktetlabs.ru>
>>>>>>> Signed-off-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
>>>>>>> Reviewed-by: Andy Moreton <amoreton@xilinx.com>
>>>>>>> ---
>>>>>>>  lib/ethdev/ethdev_driver.h | 42 ++++++++++++++++++++++++++++++++++++--
>>>>>>>  1 file changed, 40 insertions(+), 2 deletions(-)
>>>>>>>
>>>>>>> diff --git a/lib/ethdev/ethdev_driver.h b/lib/ethdev/ethdev_driver.h
>>>>>>> index 40e474aa7e..c89eefcc42 100644
>>>>>>> --- a/lib/ethdev/ethdev_driver.h
>>>>>>> +++ b/lib/ethdev/ethdev_driver.h
>>>>>>> @@ -187,11 +187,28 @@ typedef int (*eth_xstats_get_t)(struct rte_eth_dev *dev,
>>>>>>>  	struct rte_eth_xstat *stats, unsigned int n);
>>>>>>>  /**< @internal Get extended stats of an Ethernet device. */
>>>>>>>  
>>>>>>> +/**
>>>>>>> + * @internal
>>>>>>> + * Get extended stats of an Ethernet device.
>>>>>>> + *
>>>>>>> + * @param dev
>>>>>>> + *   ethdev handle of port.
>>>>>>> + * @param ids
>>>>>>> + *   IDs array to retrieve specific statistics. Must not be NULL.
>>>>>>> + * @param values
>>>>>>> + *   A pointer to a table to be filled with device statistics values.
>>>>>>> + *   Must not be NULL.
>>>>>>> + * @param n
>>>>>>> + *   Element count in @p ids and @p values.
>>>>>>> + *
>>>>>>> + * @return
>>>>>>> + *   - A number of filled in stats.
>>>>>>> + *   - A negative value on error.
>>>>>>> + */
>>>>>>>  typedef int (*eth_xstats_get_by_id_t)(struct rte_eth_dev *dev,
>>>>>>>  				      const uint64_t *ids,
>>>>>>>  				      uint64_t *values,
>>>>>>>  				      unsigned int n);
>>>>>>> -/**< @internal Get extended stats of an Ethernet device. */
>>>>>>>  
>>>>>>>  /**
>>>>>>>   * @internal
>>>>>>> @@ -218,10 +235,31 @@ typedef int (*eth_xstats_get_names_t)(struct rte_eth_dev *dev,
>>>>>>>  	struct rte_eth_xstat_name *xstats_names, unsigned int size);
>>>>>>>  /**< @internal Get names of extended stats of an Ethernet device. */
>>>>>>>  
>>>>>>> +/**
>>>>>>> + * @internal
>>>>>>> + * Get names of extended stats of an Ethernet device.
>>>>>>> + * For name count, set @p xstats_names and @p ids to NULL.
>>>>>>
>>>>>> Why limiting this behavior to 'xstats_get_names_by_id'?
>>>>>>
>>>>>> Internally 'xstats_get_names_by_id' is used to get the count, but I think this
>>>>>> is not intentionally selected, just one of the xstats_*_by_id dev_ops used.
>>>>>>
>>>>>> I think it is confusing to have this support for one of the '_by_id' dev_ops but
>>>>>> not for other. Why not require both to support returning 'count'?
>>>>>
>>>>> Simply because it is dead code. There is no point to require
>>>>> from driver to have dead code.
>>>>>
>>>>
>>>> Let me step back a little, both ethdev APIs can be used to return xstats count
>>>> by providing 'values/names' & 'ids' pointers as NULL and 'size' as 0:
>>>> 'rte_eth_xstats_get_names_by_id()'
>>>> 'rte_eth_xstats_get_by_id()'
>>>>
>>>> But internally both APIs use 'xstats_get_names_by_id' dev_ops to get the count,
>>>> as said above I believe this selection is done unintentionally.
>>>>
>>>>
>>>> I am for below two options:
>>>>
>>>> a) Internally use 'xstats_get_names' || 'xstats_get' dev_ops to get the xstats
>>>> count, and doesn't support getting xstats count for both '_by_id' dev_ops, this
>>>> simplifies driver code. As far as I remember I suggested this before, still I
>>>> prefer this one.
>>>>
>>>> b) If we will support getting xstats count from '_by_id' dev_ops, I think both
>>>> should support it, to not make it more complex to figure out which one support
>>>> what. As sample both 'xstats_get_names' and 'xstats_get' supports getting xstats
>>>> count, not just one.
>>>>
>>>
>>> In (b) do you suggest to change ethdev to use xstats_get_by_id
>>> driver op if users asks for a number of xstats using
>>> rte_eth_xstats_get_by_id()? It will complicate ethdev and
>>> will complicate drivers. Just for the symmetry?
>>>
>>
>> Not just for symmetry, but simplify the logic. Both dev_ops are very similar and
> 
> I'm sorry, but could you point of which logic you'd
> like to simply. Less requirements on driver ops
> means less code required inside.
> 
>> they are having different behavior with same parameters is confusing.
> 
> Ah, logic from PMD maintainer point of view. Does it
> really worse to require extra code inside because of it?
> 
>> If you want to simplify the drivers why not go with option (a)? Option (a) also
>> doesn't change external API, and has only minor change in the ethdev layer.
> 
> Is two extra patches in v6 (discussed below) a step
> towards (a)?
> 

I am not sure of it, for (a) ethdev layer will handle getting all stats or
getting count using 'xstats_get_names' || 'xstats_get', and '*_by_id()' dev_ops
will become simpler since they won't accept NULL 'ids' & 'values'.

Also when dev_ops merged, it forces PMDs to implement the getting by ids, but
now it is covered by ethdev layer for the PMDs that doesn't implement '_by_id()'.

>>> The patch does not change external API, does not change etcdev
>>> bahaviour. It just clarify requirements on driver ops to
>>> allow to check less in all drivers and support less cases
>>> in all drivers.
>>>
>>
>> It is not clarifying the requirements of dev_ops, but changing it. Previously
>> there was nothing saying only one of the '_by_id' dev_ops should support
>> returning element count.
> 
> I say "clarifying" since I adjust it a real source of
> truth for internals - implementation, usage of these
> callback by ethdev layer.
> 
> Yes, I agree that it changes documentation.
> 
>>
>>> If we make a one more step back, frankly speaking I think we
>>> have too many functions to retrieve statistics. I can
>>> understand from ethdev API point of view taking API stability
>>> into account etc. But why do we have so many complicated
>>> driver callbacks?
>>>> First of all I'd like to do one more cleanup:
>>> change eth_xstats_get_names_by_id_t prototype to
>>> have ids before xstats_names.
>>> I.e. eth_xstats_get_by_id_t(dev, ids, values, size)
>>> eth_xstats_get_names_by_id_t(dev, ids, names, size)
>>>
>>
>> +1
> 
> See patch 3/4 in v6
> 
>>
>>> Second, merge eth_xstats_get_names_t and eth_xstats_get_names_by_id_t
>>> callbacks to keep
>>> name of the first, but prototype from the second.
>>> The reason is equal functionality if ids==NULL,
>>> shorter name of the first and optional ids (i.e. no
>>> reason to mention optional parameter in name).
>>> Drivers which do not implement by_id_t,
>>> but implement eth_xstats_get_names_t, will simply
>>> return ENOTSUP if ids!=NULL.
>>>
>>
>> No objection, _by_id() version is already superset of the other.
> 
> See patch 4/4 in v6
> 
>>
>>> The case of values ops is more complicated,
>>> however since:
>>>
>>> 2834  * There is an assumption that 'xstat_names' and 'xstats' arrays
>>> are matched
>>> 2835  * by array index:
>>> 2836  *  xstats_names[i].name => xstats[i].value
>>> 2837  *
>>> 2838  * And the array index is same with id field of 'struct rte_eth_xstat':
>>> 2839  *  xstats[i].id == i
>>> 2840  *
>>> 2841  * This assumption makes key-value pair matching less flexible but
>>> simpler.
>>>
>>> we can switch to eth_xstats_get_by_id_t only callback as
>>> well and fill in stats->id equal to index on ethdev layer.
>>
>> When ids != NULL, the index from 'ids' can be used, isn't it.
> 
> Yes, of course, but above documentation is for API without IDs.
> 
>>
>>> However, it will require extra buffer for
>>> uint64_t *values and copying in the rte_eth_xstats_get()
>>> implementation. So, I doubt in this case.
>>>
>>
>> Overall merging xstats _by_id APIs doesn't look bad idea, but since it will
>> require change in applications, I am not really sure if benefit worth the
>> trouble it brings to users.
> 
> Yes, I agree. I'm not going to change external API.
> 
>>> In fact, it is sad that we still do not move forward
>>> in accordance with Thomas presentation made 2 years ago.
>>>
>>
>> In my experience things don't move forward without proper plan (who, what, when).
>>
> 
> +1
>