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Date:   Tue, 11 Apr 2023 11:18:07 -0700
From:   Jakub Kicinski <kuba@kernel.org>
To:     Vladimir Oltean <vladimir.oltean@nxp.com>
Cc:     netdev@vger.kernel.org, "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Paolo Abeni <pabeni@redhat.com>,
        Jamal Hadi Salim <jhs@mojatatu.com>,
        Cong Wang <xiyou.wangcong@gmail.com>,
        Jiri Pirko <jiri@resnulli.us>,
        Vinicius Costa Gomes <vinicius.gomes@intel.com>,
        Kurt Kanzenbach <kurt@linutronix.de>,
        Gerhard Engleder <gerhard@engleder-embedded.com>,
        Amritha Nambiar <amritha.nambiar@intel.com>,
        Ferenc Fejes <ferenc.fejes@ericsson.com>,
        Xiaoliang Yang <xiaoliang.yang_1@nxp.com>,
        Roger Quadros <rogerq@kernel.org>,
        Pranavi Somisetty <pranavi.somisetty@amd.com>,
        Harini Katakam <harini.katakam@amd.com>,
        Giuseppe Cavallaro <peppe.cavallaro@st.com>,
        Alexandre Torgue <alexandre.torgue@foss.st.com>,
        Michael Sit Wei Hong <michael.wei.hong.sit@intel.com>,
        Mohammad Athari Bin Ismail <mohammad.athari.ismail@intel.com>,
        Oleksij Rempel <linux@rempel-privat.de>,
        Jacob Keller <jacob.e.keller@intel.com>,
        linux-kernel@vger.kernel.org, Ferenc Fejes <fejes@inf.elte.hu>,
        Simon Horman <simon.horman@corigine.com>
Subject: Re: [PATCH v4 net-next 6/9] net/sched: mqprio: allow per-TC user
 input of FP adminStatus
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On Tue, 11 Apr 2023 20:01:51 +0300 Vladimir Oltean wrote:
> > > +	int err, tc;
> > > +
> > > +	err = nla_parse_nested(tb, TCA_MQPRIO_TC_ENTRY_MAX, opt,
> > > +			       mqprio_tc_entry_policy, extack);
> > > +	if (err < 0)
> > > +		return err;
> > > +
> > > +	if (!tb[TCA_MQPRIO_TC_ENTRY_INDEX]) {
> > > +		NL_SET_ERR_MSG(extack, "TC entry index missing");  
> > 
> > Are you not using NL_REQ_ATTR_CHECK() because iproute can't actually
> > parse the result? :(  
> 
> I could use it though.. let's assume that iproute2 is "reference code"
> and gets the nlattr structure right. Thus, the NLMSGERR_ATTR_MISS_NEST
> would be of more interest for custom user programs.
> 
> Speaking of which, is there any reference example of how to use
> NLMSGERR_ATTR_MISS_NEST? My search came up empty handed:
> https://github.com/search?p=1&q=NLMSGERR_ATTR_MISS_NEST&type=Code

Only YNL to my knowledge, basic support in the in-tree Python:
https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/tree/tools/net/ynl/lib/ynl.py#n198
And fuller support in the user space hacks which never made it out
of my GitHub:
https://github.com/kuba-moo/linux/blob/ynl-user-c-wip/tools/net/ynl/lib/ynl.c#L163

> I usually steal from hostap's error_handler(), but it looks like it
> hasn't gotten that advanced yet as to re-parse the netlink message to
> understand the reason why it got rejected.