netdev - RE: Re: [next-queue RFC 0/4] ethtool: Add support for frame preemption

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Date:   Tue, 19 May 2020 09:43:27 -0700
From:   Vinicius Costa Gomes <vinicius.gomes@...el.com>
To:     Po Liu <po.liu@....com>, Michal Kubecek <mkubecek@...e.cz>,
        "netdev\@vger.kernel.org" <netdev@...r.kernel.org>
Cc:     "intel-wired-lan\@lists.osuosl.org" 
        <intel-wired-lan@...ts.osuosl.org>,
        "jeffrey.t.kirsher\@intel.com" <jeffrey.t.kirsher@...el.com>,
        Vladimir Oltean <vladimir.oltean@....com>,
        "m-karicheri2\@ti.com" <m-karicheri2@...com>,
        "Jose.Abreu\@synopsys.com" <Jose.Abreu@...opsys.com>
Subject: RE: Re: [next-queue RFC 0/4] ethtool: Add support for frame preemption

Po Liu <po.liu@....com> writes:

> Hi Vinicius,
>
>> -----Original Message-----
>> From: Vinicius Costa Gomes <vinicius.gomes@...el.com>
>> Sent: 2020年5月19日 3:34
>> To: Michal Kubecek <mkubecek@...e.cz>; netdev@...r.kernel.org
>> Cc: intel-wired-lan@...ts.osuosl.org; jeffrey.t.kirsher@...el.com; Vladimir
>> Oltean <vladimir.oltean@....com>; Po Liu <po.liu@....com>; m-
>> karicheri2@...com; Jose.Abreu@...opsys.com
>> Subject: Re: [next-queue RFC 0/4] ethtool: Add support for frame
>> preemption
>> 
>> Hi,
>> 
>> Michal Kubecek <mkubecek@...e.cz> writes:
>> 
>> > On Fri, May 15, 2020 at 06:29:44PM -0700, Vinicius Costa Gomes wrote:
>> >> Hi,
>> >>
>> >> This series adds support for configuring frame preemption, as defined
>> >> by IEEE 802.1Q-2018 (previously IEEE 802.1Qbu) and IEEE 802.3br.
>> >>
>> >> Frame preemption allows a packet from a higher priority queue marked
>> >> as "express" to preempt a packet from lower priority queue marked as
>> >> "preemptible". The idea is that this can help reduce the latency for
>> >> higher priority traffic.
>> >>
>> >> Previously, the proposed interface for configuring these features was
>> >> using the qdisc layer. But as this is very hardware dependent and all
>> >> that qdisc did was pass the information to the driver, it makes sense
>> >> to have this in ethtool.
>> >>
>> >> One example, for retrieving and setting the configuration:
>> >>
>> >> $ ethtool $ sudo ./ethtool --show-frame-preemption enp3s0 Frame
>> >> preemption settings for enp3s0:
>> >>      support: supported
>> >
>> > IMHO we don't need a special bool for this. IIUC this is not a state
>> > flag that would change value for a particular device; either the
>> > device supports the feature or it does not. If it does not, the
>> > ethtool_ops callbacks would return -EOPNOTSUPP (or would not even
>> > exist if the driver has no support) and ethtool would say so.
>> 
>> (I know that the comments below only apply if "ethtool-way" is what's
>> decided)
>> 
>> Cool. Will remove the supported bit.
>
> Shall it move to the link_ksettings fixed supported list? So can be
> checked by the ethtool -k command. I understand that the hw features
> are encouraged listing in the ksettings.

Having it in link_ksettings might make sense, using something like
"frame-preemption: off [fixed]" to mean "not supported" sounds nice.

> The two MACs should all be initialized at driver size. And all frame queues should assigned to the express MAC by default. That looks as normal mode called preemption disable.
> Any frame queues assigned pass preemptable MAC could be called
> preemption enable.

If you mean to have frame-preemption enabled by default, I think it
should be a per-driver decision, and probably disabled by default, at
least in the begining: frame preemption might interact badly with other
features, jumbo frames and EEE come to mind.


Cheers,
--
Vinicius