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

[Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>]

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?

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.

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)

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.

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.
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.

In fact, it is sad that we still do not move forward
in accordance with Thomas presentation made 2 years ago.