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Date:   Tue, 6 Jun 2023 12:16:31 +0300
From:   Vladimir Oltean <vladimir.oltean@nxp.com>
To:     wei.fang@nxp.com
Cc:     claudiu.manoil@nxp.com, davem@davemloft.net, edumazet@google.com,
        kuba@kernel.org, pabeni@redhat.com, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH net] net: enetc: correct the indexes of highest and 2nd
 highest TCs
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References: <20230606084618.1126471-1-wei.fang@nxp.com>
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On Tue, Jun 06, 2023 at 04:46:18PM +0800, wei.fang@nxp.com wrote:
> From: Wei Fang <wei.fang@nxp.com>
> 
> For ENETC hardware, the TCs are numbered from 0 to N-1, where N
> is the number of TCs. Numerically higher TC has higher priority.
> It's obvious that the highest priority TC index should be N-1 and
> the 2nd highest priority TC index should be N-2.
> However, the previous logic uses netdev_get_prio_tc_map() to get
> the indexes of highest priority and 2nd highest priority TCs, it
> does not make sense and is incorrect. It may get wrong indexes of
> the two TCs and make the CBS unconfigurable. e.g.

Well, you need to consider that prior to commit 1a353111b6d4 ("net:
enetc: act upon the requested mqprio queue configuration"), the driver
would always set up an identity mapping between priorities, traffic
classes, rings and netdev queues.

So, yes, giving a "tc" argument to netdev_get_prio_tc_map() is
semantically incorrect, but it only started being a problem when the
identity mapping started being configurable.

> $ tc qdisc add dev eno0 parent root handle 100: mqprio num_tc 6 \
> 	map 0 0 1 1 2 3 4 5 queues 1@0 1@1 1@2 1@3 2@4 2@6 hw 1
> $ tc qdisc replace dev eno0 parent 100:6 cbs idleslope 100000 \
> 	sendslope -900000 hicredit 12 locredit -113 offload 1
> $ Error: Specified device failed to setup cbs hardware offload.
>   ^^^^^

ok.

> 
> Fixes: c431047c4efe ("enetc: add support Credit Based Shaper(CBS) for hardware offload")

In principle, there shouldn't be an issue with backporting the fix that
far (v5.5), even if it is unnecessary beyond commit 1a353111b6d4 (v6.3).
If you want to respin the patch to clarify the situation, fine. If not,
also fine.

> Signed-off-by: Wei Fang <wei.fang@nxp.com>
> ---

Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>