Re: [PATCH 0/3] support setting lanes

[Thomas Monjalon <thomas@monjalon.net>]

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.