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        Claudiu Manoil <claudiu.manoil@nxp.com>,
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        Jerry Huang <jerry.huang@nxp.com>, Leo Li <leoyang.li@nxp.com>,
        Po Liu <po.liu@nxp.com>
From:   Andre Guedes <andre.guedes@linux.intel.com>
Subject: Re: [v1,net-next, 1/2] ethtool: add setting frame preemption of traffic classes
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        "allison@lohutok.net" <allison@lohutok.net>,
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Date:   Tue, 10 Dec 2019 18:52:49 -0800
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Hi Po,

Quoting Po Liu (2019-11-27 01:59:18)
> IEEE Std 802.1Qbu standard defined the frame preemption of port
> traffic classes. This patch introduce a method to set traffic
> classes preemption. Add a parameter 'preemption' in struct
> ethtool_link_settings. The value will be translated to a binary,
> each bit represent a traffic class. Bit "1" means preemptable
> traffic class. Bit "0" means express traffic class.  MSB represent
> high number traffic class.
> 
> If hardware support the frame preemption, driver could set the
> ethernet device with hw_features and features with NETIF_F_PREEMPTION
> when initializing the port driver.
> 
> User can check the feature 'tx-preemption' by command 'ethtool -k
> devname'. If hareware set preemption feature. The property would
> be a fixed value 'on' if hardware support the frame preemption.
> Feature would show a fixed value 'off' if hardware don't support
> the frame preemption.
> 
> Command 'ethtool devname' and 'ethtool -s devname preemption N'
> would show/set which traffic classes are frame preemptable.
> 
> Port driver would implement the frame preemption in the function
> get_link_ksettings() and set_link_ksettings() in the struct ethtool_ops.

In an early RFC series [1], we proposed a way to support frame preemption. I'm not
sure if you have considered it before implementing this other proposal based on
ethtool interface so I thought it would be a good idea to bring that up to your
attention, just in case.

In that initial proposal, Frame Preemption feature is configured via taprio
qdisc. For example:

$ tc qdisc add dev IFACE parent root handle 100 taprio \
      num_tc 3 \
      map 2 2 1 0 2 2 2 2 2 2 2 2 2 2 2 2 \
      queues 1@0 1@1 2@2 \
      preemption 0 1 1 1 \
      base-time 10000000 \
      sched-entry S 01 300000 \
      sched-entry S 02 300000 \
      sched-entry S 04 400000 \
      clockid CLOCK_TAI

It also aligns with the gate control operations Set-And-Hold-MAC and
Set-And-Release-MAC that can be set via 'sched-entry' (see Table 8.7 from
802.1Q-2018 for further details.

Please share your thoughts on this.

Regards,

Andre