Re: [PATCH 1/6] net: phy: add interface modes for XFI, SFI

[Russell King - ARM Linux admin <linux@armlinux.org.uk>]

On Thu, Dec 19, 2019 at 09:34:57PM +0000, Madalin Bucur (OSS) wrote:
> > -----Original Message-----
> > From: Russell King - ARM Linux admin <linux@armlinux.org.uk>
> > Sent: Thursday, December 19, 2019 9:03 PM
> > To: Madalin Bucur <madalin.bucur@nxp.com>
> > Cc: davem@davemloft.net; netdev@vger.kernel.org; andrew@lunn.ch;
> > f.fainelli@gmail.com; hkallweit1@gmail.com; shawnguo@kernel.org;
> > devicetree@vger.kernel.org
> > Subject: Re: [PATCH 1/6] net: phy: add interface modes for XFI, SFI
> > 
> > On Thu, Dec 19, 2019 at 06:32:51PM +0000, Madalin Bucur wrote:
> > > > -----Original Message-----
> > > > From: Russell King - ARM Linux admin <linux@armlinux.org.uk>
> > > > Sent: Thursday, December 19, 2019 7:29 PM
> > > > To: Madalin Bucur <madalin.bucur@nxp.com>
> > > > Cc: davem@davemloft.net; netdev@vger.kernel.org; andrew@lunn.ch;
> > > > f.fainelli@gmail.com; hkallweit1@gmail.com; shawnguo@kernel.org;
> > > > devicetree@vger.kernel.org
> > > > Subject: Re: [PATCH 1/6] net: phy: add interface modes for XFI, SFI
> > > >
> > > > On Thu, Dec 19, 2019 at 05:21:16PM +0200, Madalin Bucur wrote:
> > > > > From: Madalin Bucur <madalin.bucur@nxp.com>
> > > > >
> > > > > Add explicit entries for XFI, SFI to make sure the device
> > > > > tree entries for phy-connection-type "xfi" or "sfi" are
> > > > > properly parsed and differentiated against the existing
> > > > > backplane 10GBASE-KR mode.
> > > >
> > > > 10GBASE-KR is actually used for XFI and SFI (due to a slight mistake
> > on
> > > > my part, it should've been just 10GBASE-R).
> > > >
> > > > Please explain exactly what the difference is between XFI, SFI and
> > > > 10GBASE-R. I have not been able to find definitive definitions for
> > > > XFI and SFI anywhere, and they appear to be precisely identical to
> > > > 10GBASE-R. It seems that it's just a terminology thing, with
> > > > different groups wanting to "own" what is essentially exactly the
> > > > same interface type.
> > >
> > > Hi Russell,
> > >
> > > 10GBase-R could be used as a common nominator but just as well 10G and
> > > remove the rest while we're at it. There are/may be differences in
> > > features, differences in the way the HW is configured (the most
> > > important aspect) and one should be able to determine what interface
> > > type is in use to properly configure the HW. SFI does not have the CDR
> > > function in the PMD, relying on the PMA signal conditioning vs the XFI
> > > that requires this in the PMD. We kept the xgmii compatible for so long
> > > without much issues until someone started cleaning up the PHY supported
> > > modes. Since we're doing that, let's be rigorous. The 10GBase-KR is
> > > important too, we have some backplane code in preparation and having it
> > > there could pave the way for a simpler integration.
> > 
> > The problem we currently have is:
> > 
> > $ grep '10gbase-kr' arch/*/boot/dts -r
> > 
> > virtually none of those are actually backplane. For the mcbin matches,
> > these are either to a 88x3310 PHY for the doubleshot, which dynamically
> > operates between XFI, 5GBASE-R, 2500BASE-X, or SGMII according to the
> > datasheet.
> 
> Yes, I've seen it's used already in several places:
> 
> $ grep PHY_INTERFACE_MODE_10GKR drivers/net -nr
> drivers/net/phy/marvell10g.c:219:       if (iface != PHY_INTERFACE_MODE_10GKR) {
> drivers/net/phy/marvell10g.c:307:           phydev->interface != PHY_INTERFACE_MODE_10GKR)
> drivers/net/phy/marvell10g.c:389:            phydev->interface == PHY_INTERFACE_MODE_10GKR) && phydev->link) {
> drivers/net/phy/marvell10g.c:398:                       phydev->interface = PHY_INTERFACE_MODE_10GKR;
> drivers/net/phy/phylink.c:296:          case PHY_INTERFACE_MODE_10GKR:
> drivers/net/phy/aquantia_main.c:361:            phydev->interface = PHY_INTERFACE_MODE_10GKR;
> drivers/net/phy/aquantia_main.c:499:        phydev->interface != PHY_INTERFACE_MODE_10GKR)
> drivers/net/phy/sfp-bus.c:340:          return PHY_INTERFACE_MODE_10GKR;
> drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c:1117:   return interface == PHY_INTERFACE_MODE_10GKR ||
> drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c:1203:   case PHY_INTERFACE_MODE_10GKR:
> drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c:1652:   case PHY_INTERFACE_MODE_10GKR:
> drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c:4761:   case PHY_INTERFACE_MODE_10GKR:
> drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c:4783:   case PHY_INTERFACE_MODE_10GKR:
> 
> We should fix this, if it's incorrect.
> 
> > If we add something else, then the problem becomes what to do about
> > that lot - one of the problems is, it seems we're going to be breaking
> > DT compatibility by redefining 10gbase-kr to be correct.
> 
> We need the committer/maintainer to update that to a correct value.

The general principle is, we don't break existing DT - in that, we
expect DT files from current kernels to work with future kernels. So,
we're kind of stuck with "10gbase-kr" being used for this at least in
the medium term.

By all means introduce "xfi" and "sfi" if you think that there is a
need to discriminate between the two, but I've seen no hardware which
that treats them any differently from 10gbase-r.

If we want to support real 10gbase-kr, then I think we need to consider
how to do that without affecting compatibility with what we already
have.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTC broadband for 0.8mile line in suburbia: sync at 12.1Mbps down 622kbps up
According to speedtest.net: 11.9Mbps down 500kbps up