linux-kernel - Re: [v1,net-next, 1/2] ethtool: add setting frame preemption of traffic classes

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Date:   Tue, 10 Dec 2019 18:52:49 -0800
From:   Andre Guedes <andre.guedes@...ux.intel.com>
To:     "alexandru.ardelean@...log.com" <alexandru.ardelean@...log.com>,
        "allison@...utok.net" <allison@...utok.net>,
        "andrew@...n.ch" <andrew@...n.ch>,
        "ayal@...lanox.com" <ayal@...lanox.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "f.fainelli@...il.com" <f.fainelli@...il.com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "hauke.mehrtens@...el.com" <hauke.mehrtens@...el.com>,
        "hkallweit1@...il.com" <hkallweit1@...il.com>,
        "jiri@...lanox.com" <jiri@...lanox.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "pablo@...filter.org" <pablo@...filter.org>,
        "saeedm@...lanox.com" <saeedm@...lanox.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>, Po Liu <po.liu@....com>
Cc:     "vinicius.gomes@...el.com" <vinicius.gomes@...el.com>,
        "simon.horman@...ronome.com" <simon.horman@...ronome.com>,
        Claudiu Manoil <claudiu.manoil@....com>,
        Vladimir Oltean <vladimir.oltean@....com>,
        Alexandru Marginean <alexandru.marginean@....com>,
        Xiaoliang Yang <xiaoliang.yang_1@....com>,
        Roy Zang <roy.zang@....com>, Mingkai Hu <mingkai.hu@....com>,
        Jerry Huang <jerry.huang@....com>, Leo Li <leoyang.li@....com>,
        Po Liu <po.liu@....com>
Subject: Re: [v1,net-next, 1/2] ethtool: add setting frame preemption of traffic classes

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