RE: [RFC net-next 2/5] net: phylink: add separate pcs operations structure

[Jose Abreu <Jose.Abreu@synopsys.com>]

From: Russell King - ARM Linux admin <linux@armlinux.org.uk>
Date: Mar/17/2020, 16:52:08 (UTC+00:00)

> On Tue, Mar 17, 2020 at 04:04:28PM +0000, Jose Abreu wrote:
> > From: Russell King - ARM Linux admin <linux@armlinux.org.uk>
> > Date: Mar/17/2020, 15:56:10 (UTC+00:00)
> > 
> > > > Please consider removing this condition and just rely on an_enabled field. 
> > > > I have USXGMII support for Clause 73 Autoneg so this won't work with 
> > > > that.
> 
> Do you really mean USXGMII or XLGMII that you recently contributed?
> XLGMII makes more sense for clause 73.
> 
> > > That is actually incorrect.  SGMII can have an_enabled being true, but
> > > SGMII is not an autonegotiation between the MAC and PHY - it is merely
> > > a mechanism for the PHY to inform the MAC what the results of _its_
> > > negotiation are.
> > > 
> > > I suspect USXGMII is the same since it is just an "upgraded" version of
> > > SGMII.  Please can you check whether there really is any value in trying
> > > (and likely failing) to restart the "handshake" with the PHY from the
> > > MAC end, rather than requesting the PHY to restart negotiation on its
> > > media side.
> > 
> > I think we are speaking of different things here. I'm speaking about 
> > end-to-end Autoneg. Not PHY <-> PCS <-> MAC.
> 
> What do you mean end-to-end autoneg?  Let's take a simple example for
> SGMII, here is the complete setup:
> 
> MAC <--> PCS <--SGMII--> PHY <--MEDIA--> PHY <--SGMII--> MAC
> 
> Generally, asking the PCS to "renegotiate" will either be ignored, or
> might cause the PCS to restart the handshake with the PHY depending on
> the implementation.  It will not cause the PHY to renegotiate with the
> remote PHY.
> 
> Asking the PHY to renegotiate will cause the link to drop, which
> updates the config_reg word sent to the PCS to indicate link down.
> When the link is re-established, the config_reg word is updated again
> with the new link parameters and that sent to the PCS.
> 
> So, just because something is closer to the MAC does not necessarily
> mean that it causes more of the blocks to "renegotiate" when asked to
> do so.
> 
> In SGMII, the PHY is the "master" and this is what needs negotiation
> restarted on "ethtool -r" to have the effect that the user desires.
> 
> I believe (but I don't know for certain, because the USXGMII
> documentation is not available to me) that USXGMII is SGMII extended
> up to additionally include 10G, 5G and 2.5G speeds, and otherwise is
> basically the same mechanism.
> 
> So, I would suggest that USXGMII and SGMII should be treated the same,
> and for both of them, a renegotiation should always be performed at
> the PHY and not the PCS.
> 
> There is another reason not to try triggering renegotiation at both
> the PHY and PCS.  When the PHY is renegotiating, the state machines
> at both the PHY and PCS end are in the process of changing - if we
> hit the PCS with a request to renegotiate, and the hardware isn't
> setup to ignore it, that could occur in an unexpected state - the risk
> of triggering a hardware problem could be higher.
> 
> So, based on this, I don't think it's a good idea to restart
> negotiation at the PCS for SGMII and USXGMII modes.
> 
> For the 802.3z based protocols, it makes complete sense to do so,
> because the PCS are the blocks involved in the actual media negotiation
> and there is no place else to restart negotiation:
> 
> MAC <---> PCS <----fiber 1000base-X----> PCS <---> MAC

That's kind of the setup I have, hence the need for me to restart ... I 
have this:

MAC <-> PCS <-> SERDES <-> Copper <-> SERDES <-> PCS <-> MAC

So, no PHY to restart Autoneg.

> 
> > I'm so sorry but I'm not an expert in this field, I just deal mostly with 
> > IP.
> > 
> > Anyway, I'm speaking about end-to-end Clause 73 Autoneg which involves 
> > exchanging info with the peer. If peer for some reason is not available to 
> > receive this info then AutoNeg will not succeed. Hence the reason to 
> > restart it.
> 
> Clause 73 covers backplane and copper cable, and isn't USXGMII.
> In the case of copper, I would expect the setup to be very similar
> to what I've outlined above for the SGMII case, but using USXGMII
> instead of SGMII, or automatically selecting something like
> 10GBASE-R, 5GBASE-R, 2500BASE-X, or SGMII depending on the result
> from copper negotiation.  Depending on the Ethernet PHY being used,
> it may or may not have the in-band control_reg word even for SGMII.
> In any case, what I've said above applies: to provoke end-to-end
> renegotiation, the PHY needs to be restarted, not the MAC's PCS.
> 
> For backplane, things are a little different, and that may indeed
> require the MAC's PCS to be restarted, and for that case it may be
> reasonable to expand the conditional there.

Yes, I think it makes sense due to the setup I outlined above.

> However, note that for the purposes of this patch, no change of
> behaviour over the current behaviour is intended; a change to this
> will need to be a separate patch.
> 
> Thanks.

Understood. Thanks for such thoughtfully explanation!

---
Thanks,
Jose Miguel Abreu