Re: [PATCH net-next 2/2] net: pcs: xpcs: select operating mode for 10G-baseR capable PCS

[Andrew Lunn <andrew@lunn.ch>]

On Fri, Oct 16, 2026 at 03:30:56PM +0100, Daniel Thompson wrote:
> On Thu, Oct 15, 2026 at 11:09:14PM +0200, Andrew Lunn wrote:
> > I don't know this PCS driver, so maybe dumb questions....
> >
> > > Currently the XPCS found on Toshiba TC9564 is unable to operate at
> > > 2500base-X and slower with a PHY connected using SGMII/2500base-X
> > > (in our case a Qualcomm QCA8081).
> >
> > Is the PHY actually relevant here? What about SGMII/2500base-X for an
> > SFP?
> 
> No, I don't think the phy is relevant here (the bit in brackets just
> records the specific hardware combination that we observed the problem
> on).
> 
> 
> > Is this purely a PCS problem, or does what the PCS connect to make a
> > difference?
> 
> AFAIK it is purely a PCS problem (but it's all soldered down so I have
> no chance to prove I am right by testing it).

Thanks. Given it was stated i was thinking it could be relevant. But
how can it be relevant? And then i thought that the PCS only supports
10Gbase-R, and the PHY is using rate adaptation to handle slower media
speeds?

That seemed unlikely.
 
> > > The problem arises because this XPCS supports 10Gbase-R. That means
> > > that the reset value of SR_XS_PCS_CTRL2:PCS_TYPE_SEL (0) is valid and
> > > this suppresses the modal switching based on bit 13 of SR_PMA_CTRL1
> > > or SR_XS_PCS_CTRL1.
> >
> > ~/linux/drivers/net/pcs$ grep SR_XS_PCS_CTRL1 *
> > ~/linux/drivers/net/pcs$ grep SR_PMA_CTRL1 *
> > ~/linux/drivers/net/pcs$
> >
> > This is not helping me understand what is going on here.
> 
> These names are exactly as they appear in the technical documentation
> for the TC9564. I assumed these to be inherited from the Synopsis XPCS
> docs and would make sense in the context of the XPCS driver.

I however don't have this documentation, just the driver, and 802.3.

> However, now you mention it I can see that "Speed selection bit (13) of
> PCS control 1" would be considerably clearer for reviewers who take a
> wider view! Will fix this and will also be a bit clearer about how XPCS
> treats PCS_TYPE_SEL.
> 
> 
> > > +	if (mdio_stat2 & MDIO_PCS_STAT2_10GBR) {
> > > +		switch (interface) {
> > > +		case PHY_INTERFACE_MODE_SGMII:
> > > +		case PHY_INTERFACE_MODE_1000BASEX:
> > > +		case PHY_INTERFACE_MODE_2500BASEX:
> > > +			/*
> > > +			 * Why are we writing MDIO_PCS_CTRL2_TYPE + 1? We want
> > > +			 * the modal behaviour that comes when we pick a
> > > +			 * reserved value. XPCS allocates extra bits to this
> > > +			 * field and allocates values from 15 down so
> > > +			 * MDIO_PCS_CTRL2_TYPE + 1 is the value likely to be
> > > +			 * allocated last (and hopefully never).
> >
> > MDIO_PCS_CTRL2_TYPE is 3. Looking at 802.3, the value 4 is
> > 40GBASE-R. The 2022 version of the standard has values defined up to
> > 10011b, for speeds up to 400G,
> >
> > Alex added support for 25G, 50G, 100G.
> >
> > Is this an issue?
> 
> It will probably be an issue when the kernel is first called up on to
> support an XPCS that supports 40GBASE-R.

Which is likely to happen sometime in the future. Maybe Alex's
hardware even supports it, but if you can do 50G, why bother with 40G?

 
> The XPCS (at least in TC956x) handles MDIO_PCS_CTRL2 in a slightly odd way.
> It 4 bits wide and accepts 10GBASE-R, 10GBASE-X, 5GBASE-R and 2.5GBASE-X
> is defined in the standards up to 2018. All other bit patterns are
> reserved and instruct the XPCS to use Speed selection bit (13) of PCS
> control 1 to choose between 10GBASE-X or 1000BASE-X.

We probably need Alex in this thread, what happens for 25G, 50G, 100G?

Maybe there are different revisions of the IP, and later revisions
make this register follow the standard?

> 
> That means we *must* write a reserved value to this register because the
> default value after a reset prevents <=1G modes from working (I think
> automatic switch over between 1G and 2.5G also liked this to a reserved
> value but I don't fully recall right now).
> 
> On that basis the best bit pattern to use one that isn't implemented by
> whatever XPCS we happen to be running on. One simple approach might be
> keep using 40GBASE-R but to check PCS Status 2 to ensure we fail in a
> non-silent manner on hypothesized future XPCS hardware:
> 
> 	if (mdio_stat2 & MDIO_PCS_STAT2_10GBR) {
> 		if (mdio_stat2 & MDIO_PCS_STAT2_40GBR) {
> 			dev_err(dev, "40GBASE-R is *not* a reserved "
> 			              value on this XPCS");
> 			return -ENOTSUP;
> 		}

Maybe. But we need to ensure it does not cause regressions for fbnic,
which might actually support this speed.

	Andrew