Re: [PATCH 0/3] support setting lanes

[huangdengdui <huangdengdui@huawei.com>]

On 2024/3/21 16:28, Thomas Monjalon wrote:
> 21/03/2024 03:02, huangdengdui:
>>
>> On 2024/3/20 20:31, Ferruh Yigit wrote:
>>> On 3/18/2024 9:26 PM, Damodharam Ammepalli wrote:
>>>> On Mon, Mar 18, 2024 at 7:56 AM Thomas Monjalon <thomas@monjalon.net> wrote:
>>>>>
>>>>> 12/03/2024 08:52, Dengdui Huang:
>>>>>> Some speeds can be achieved with different number of lanes. For example,
>>>>>> 100Gbps can be achieved using two lanes of 50Gbps or four lanes of 25Gbps.
>>>>>> When use different lanes, the port cannot be up.
>>>>>
>>>>> I'm not sure what you are referring to.
>>>>> I suppose it is not PCI lanes.
>>>>> Please could you link to an explanation of how a port is split in lanes?
>>>>> Which hardware does this?
>>>>>
>>>> This is a snapshot of 100Gb that the latest BCM576xx supports.
>>>> 100Gb (NRZ: 25G per lane, 4 lanes) link speed
>>>> 100Gb (PAM4-56: 50G per lane, 2 lanes) link speed
>>>> 100Gb (PAM4-112: 100G per lane, 1 lane) link speed
>>>>
>>>> Let the user feed in lanes=< integer value> and the NIC driver decides
>>>> the matching combination speed x lanes that works. In future if a new speed
>>>> is implemented with more than 8 lanes, there wouldn't be a need
>>>> to touch this speed command. Using separate lane command would
>>>> be a better alternative to support already shipped products and only new
>>>> drivers would consider this lanes configuration, if applicable.
>>>>
>>>
>>> As far as I understand, lane is related to the physical layer of the
>>> NIC, there are multiple copies of transmitter, receiver, modulator HW
>>> block and each set called as a 'lane' and multiple lanes work together
>>> to achieve desired speed. (please correct me if this is wrong).
>>>
>>> Why not just configuring the speed is not enough? Why user needs to know
>>> the detail and configuration of the lanes?
>>> Will it work if driver/device configure the "speed x lane" internally
>>> for the requested speed?
>>>
>>> Is there a benefit to force specific lane count for a specific speed
>>> (like power optimization, just a wild guess)?
>>>
>>>
>>> And +1 for auto-negotiation if possible.
>>
>> As you said above,，multiple lanes work together to achieve desired speed.
>> For example, the following solutions can be used to implement 100G:
>> 1、Combines four 25G lanes
>> 2、Combines two 50G lanes
>> 3、A single 100G lane
>>
>> It is assumed that two ports are interconnected and the two ports support
>> the foregoing three solutions. But, we just configured the speed to 100G and
>> one port uses four 25G lanes by default and the other port uses two 50G lanes
>> by default, the port cannot be up. In this case, we need to configure the
>> two ports to use the same solutions (for example, uses two 50G lanes)
>> so that the ports can be up.
> 
> Why this config is not OK? How do we know?
> Really I have a very bad feeling about this feature.
> 
> 
Sorry, I don't quite understand your question.
Are you asking why cannot be up when one port uses four 25G lanes and the other port uses two 50G lanes?

100GBASE-SR2 (two 50G lanes) and 100GBASE-SR4 (four 25G lanes) have different standards at the physical layer.[1]
So it's not possible to communicate. Configuring lanes can help the driver choose the same standard.

[1]
https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=9844436