RE: [PATCH 1/3] ethdev: fix telemetry xstats return null with some PMDs

[Morten Brørup <mb@smartsharesystems.com>]

> From: fengchengwen [mailto:fengchengwen@huawei.com]
> Sent: Thursday, 28 April 2022 15.39
> 
> On 2022/4/25 18:16, Morten Brørup wrote:
> >> From: fengchengwen [mailto:fengchengwen@huawei.com]
> >> Sent: Sunday, 24 April 2022 05.45
> >>
> >> The root cause is: when the xstats is NULL and n less than required
> >> number of
> >> elements is zero, the return value of rte_eth_xstats_get is
> ambiguous
> >> from
> >> rte_ethdev.h's declaration.
> >>
> >> But the implementation of rte_eth_xstats_get return required number
> of
> >> elements
> >> when xstats is NULL and n less than required number of elements.
> >>
> >> There are two modification schemes:
> >> a) the value of xstats cannot be NULL, and the value of n must be
> >> greater than
> >>    or equal to the required number, otherwise, an error code is
> >> returned.
> >> b) define the behavior as the same as rte_eth_xstats_get_names,
> which
> >> means return
> >>    required number of elelement when xstats is NULL and n less than
> >> required number
> >>    of elements.
> >>
> >> I prefer the scheme a because rte_eth_xstats_get and
> >> rte_eth_xstats_get_name are
> >> symbiotic, and it's not necessary to both implement the same logic.
> >>
> >> Also for scheme a, there is no need to modify the PMD
> implementation.
> >>
> >> What about your opinions ?
> >
> > This is an excellent proposal.
> >
> > And if the documentation for rte_eth_xstats_get() refers to
> rte_eth_xstats_get_names() for getting the number of elements, it is
> perfect.
> 
> Hi Morten,
> 
>    I did a deepin review and found that many application use
> rte_eth_xstats_get(port_id, NULL, 0),
> and also many PMD already support return required number of entries
> when xstats is NULL. So in
> the v2, I use the modified hns3/ipn3ke/mvpp2/axgbe PMD method and
> explicit return value when xstats
> is NULL of rte_eth_xstats-get().

Instead of modifying the drivers, consider copying this code from rte_eth_xstats_get_names() to rte_eth_xstats_get():

	cnt_expected_entries = eth_dev_get_xstats_count(port_id);
	if (xstats_names == NULL || cnt_expected_entries < 0 ||
			(int)size < cnt_expected_entries)
		return cnt_expected_entries;

Just a suggestion. Either solution is fine with me.

> 
> Thanks.
> 
> >
> >>
> >> On 2022/4/21 14:49, Andrew Rybchenko wrote:
> >>> On 4/16/22 04:38, Stephen Hemminger wrote:
> >>>> On Sat, 16 Apr 2022 09:07:45 +0800
> >>>> Chengwen Feng <fengchengwen@huawei.com> wrote:
> >>>>
> >>>>> Currently the telemetry xstats uses rte_eth_xstats_get() to
> >> retrieve
> >>>>> the number of elements. But the value to be returned when the
> >> parameter
> >>>>> 'xstats' is NULL is not specified, some PMDs (eg.
> >> hns3/ipn3ke/mvpp2/
> >>>>> axgbe) return zero while others return the required number of
> >> elements.
> >>>>
> >>>> Lets fix the PMD's this impacts other code as well.
> >>>
> >>> +1
> >>>
> >>> .
> >>
> >
>