[PATCH net-next v3 0/5] TSN auto negotiation between 1G and 2.5G

[Choong Yong Liang]

On 21/9/2023 10:09 pm, Russell King (Oracle) wrote:
> On Thu, Sep 21, 2023 at 03:14:59PM +0200, Andrew Lunn wrote:
>>> Auto-negotiation between 10, 100, 1000Mbps will use
>>> in-band auto negotiation. Auto-negotiation between 10/100/1000Mbps and
>>> 2.5Gbps will work as the following proposed flow, the stmmac driver reads
>>> the PHY link status registers then identifies the negotiated speed.
>>
>> I don't think you replied to my comment.
>>
>> in-band is just an optimisation. It in theory allows you to avoid a
>> software path, the PHY driver talking to the MAC driver about the PHY
>> status. As an optimisation, it is optional. Linux has the software
>> path and the MAC driver you are using basically has it implemented.
> 
> Sorry Andrew, I have to disagree. It isn't always optional - there are
> PHYs out there where they won't pass data until the in-band exchange
> has completed. If you try to operate out-of-band without the PHY being
> told that is the case, and program the MAC/PCS end not to respond to
> the in-band frames from the PHY, the PHY will report link up as normal
> (since it reports the media side), but no data will flow because the
> MAC facing side of the PHY hasn't completed.
> 
> The only exception are PHYs that default to in-band but have an inband
> bypass mode also enabled to cover the case where the MAC/PCS doesn't
> respond to the inband messages.
> 
Russell is correct, we did set out-of-band for PCS and configured MAC.
Due to the PHY not being completed, there will be no data flow through.