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

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Date:   Tue, 19 May 2020 15:39:54 -0700
From:   Andre Guedes <andre.guedes@...el.com>
To:     Vinicius Costa Gomes <vinicius.gomes@...el.com>,
        intel-wired-lan@...ts.osuosl.org
Cc:     Vinicius Costa Gomes <vinicius.gomes@...el.com>,
        jeffrey.t.kirsher@...el.com, netdev@...r.kernel.org,
        vladimir.oltean@....com, po.liu@....com, m-karicheri2@...com,
        Jose.Abreu@...opsys.com
Subject: Re: [next-queue RFC 0/4] ethtool: Add support for frame preemption

Hi,

Quoting Vinicius Costa Gomes (2020-05-15 18:29:44)
> One example, for retrieving and setting the configuration:
> 
> $ ethtool $ sudo ./ethtool --show-frame-preemption enp3s0
> Frame preemption settings for enp3s0:
>         support: supported
>         active: active

IIUC the code in patch 2, 'active' is the actual configuration knob that
enables or disables the FP functionality on the NIC.

That sounded a bit confusing to me since the spec uses the term 'active' to
indicate FP is currently enabled at both ends, and it is a read-only
information (see 12.30.1.4 from IEEE 802.1Q-2018). Maybe if we called this
'enabled' it would be more clear.

>         supported queues: 0xf
>         supported queues: 0xe
>         minimum fragment size: 68

I'm assuming this is the configuration knob for the minimal non-final fragment
defined in 802.3br.

My understanding from the specs is that this value must be a multiple from 64
and cannot assume arbitrary values like shown here. See 99.4.7.3 from IEEE
802.3 and Note 1 in S.2 from IEEE 802.1Q. In the previous discussion about FP,
we had this as a multiplier factor, not absolute value.

Regards,

Andre