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

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Message-ID: <13d8b7ba-8afd-cb67-c782-56aff1412bcd@ti.com>
Date:   Wed, 20 May 2020 08:47:36 -0400
From:   Murali Karicheri <m-karicheri2@...com>
To:     Vinicius Costa Gomes <vinicius.gomes@...el.com>,
        Andre Guedes <andre.guedes@...el.com>,
        <intel-wired-lan@...ts.osuosl.org>
CC:     <jeffrey.t.kirsher@...el.com>, <netdev@...r.kernel.org>,
        <vladimir.oltean@....com>, <po.liu@....com>,
        <Jose.Abreu@...opsys.com>
Subject: Re: [next-queue RFC 0/4] ethtool: Add support for frame preemption

Hi Vinicius,

On 5/19/20 7:37 PM, Vinicius Costa Gomes wrote:
> Andre Guedes <andre.guedes@...el.com> writes:
> 
>> 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.
> 
> Good point. Will rename this to "enabled".
> 
>>
>>>          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.
> 
> I thought that exposing this as "(1 + N)*64" (with 0 <= N <= 3) that it
> was more related to what's exposed via LLDP than the actual capabilities
> of the hardware. And for the hardware I have actually the values
> supported are: (1 + N)*64 + 4 (for N = 0, 1, 2, 3).
> 
> So I thought I was better to let the driver decide what values are
> acceptable.
> 
> This is a good question for people working with other hardware.
> 
> 
AM65 CPSW supports this as a multiple of 64. Since this ethtool
configuration is for the hardware, might want to make it as a multiple
of 64.

Murali

-- 
Murali Karicheri
Texas Instruments