Re: [PATCH] ethdev: fix queue mapping documentation

[Jerin Jacob <jerin.jacob@caviumnetworks.com>]

-----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.