Subject: Re: [net-next PATCH v6] net: macb: Fix several edge cases in validate

From: "Russell King (Oracle)" <[email-masked]>
Date: Wed, 17 Nov 2021 00:22:26 +0000

On Tue, Nov 16, 2021 at 05:56:43PM -0500, Sean Anderson wrote:
> Hi Russell,
> 
> I have a few questions/comments about your tree (and pl in general).
> This is not particularly relevant to the above patch, but this is as
> good a place as any to ask.
> 
> What is the intent for the supported link modes in validate()? The docs
> say

The _link_ modes describe what gets reported to userspace via the
ethtool APIs, and therefore what appears in ethtool as the supported
and advertised capabilities for the media, whatever the "media" is
defined to be.

Generally, the "media" is what the user gets to play with to connect
two network interfaces together - so twisted pair, fibre, direct-attach
cable, etc.

> > Note that the PHY may be able to transform from one connection
> > technology to another, so, eg, don't clear 1000BaseX just
> > because the MAC is unable to BaseX mode. This is more about
> > clearing unsupported speeds and duplex settings. The port modes
> > should not be cleared; phylink_set_port_modes() will help with this.
> 
> But this is not how validate() has been/is currently implemented in many
> drivers. In 34ae2c09d46a ("net: phylink: add generic validate
> implementation"), it appears you are hewing closer to the documented
> purpose (e.g. MAC_1000FD selects all the full-duplex 1G link modes).
> Should new code try to stick to the above documentation?

I try to encourage new code to stick to this - and this is one of the
motivations behind moving to this new model, so people don't make
these kinds of mistakes.

In the case of nothing between the MAC and the media performing any
kind of speed conversion, the MAC itself doesn't have much to do with
which ethtool link modes are supported - and here's why.

Take a gigabit capable MAC that is connected via SGMII to a PHY that
supports both conventional twisted-pair media and fiber. If the
twisted-pair port is in use at 1G speeds, then we're using 1000base-T.
If the fiber port is being used, then we're using 1000base-X. The
protocol between the PHY and MAC makes no difference to what link
modes are supported.

A more extreme case could be: a 10G MAC connected to a backplane PHY
via 10G BASE-KR which is then connected to a PHY that connects to
conventional twisted-pair media.

Or a multi-speed PHY where it switches between SGMII, 2500BASE-X,
5GBASE-R, 10GBASE-R depending on the results of negotiation on the
twisted-pair media. The MAC supports operating at 10M, 100M, 1G,
2.5G, 5G, and 10G speeds, and can select between PCS that support
SGMII, 2500BASE-X, 5GBASE-R and 10GBASE-R. However, ultimately for
userspace, what matters is the media capabilities - the base-T*
ethtool link modes. 2500base-X in this situation doesn't come up
unless the PHY offers 2500base-X on the media.

The same PHY might be embedded within a SFP module, and that SFP
module might be plugged into a cage where the MAC is unable to
support the faster speeds - maybe it is only capable of up to
2.5G speeds. In which case, the system supports up to 2500BASE-T.

So you can see, the MAC side has little relevance for link modes
except for defining the speeds and duplexes that can be supported.
The type of media (-T, -X, -*R) is of no concern at this stage.

It is of little concern at the PCS except when the PCS is the
endpoint for connecting to the media (like it is in _some_ 802.3z
connections.) I say "some" because its entirely possible to use
1000base-X to talk to a PHY that connects to 1000base-T media
(and there are SFPs around that will do this by default.)

> Of course, the above leaves me quite confused about where the best place
> is to let the PCS have a say about what things are supported, and (as
> discussed below) whether it can support such a thing. The general
> perspective taken in existing drivers seems to be that PCSs are
> integrated with the MAC. This is in contrast to the IEEE specs, which
> take the pespective that the PCS is a part of the PHY. It's unclear to
> me what stance the above documentation takes.

Things can get quite complex, and I have to say the IEEE specs give
a simplified view. When you have a SGMII link to a PHY that then
connects to twisted pair media, you actually have multiple PCS:

                                 PHY
                      /----------------------\
MAC -- PCS ---<SGMII>--- PCS --- PCS --- PMA ---- media
     (sgmii)           (sgmii)   (1000baseT)

This can be seen in PHYs such as Marvell 88E151x, where the fiber
interface is re-used for SGMII, and if you read the datasheet and/or
read the fiber page registers, you find that this is being used for
the fiber side. So the PHY can be thought of two separate PHYs
back-to-back. Remember that the PCS for 1000BASE-X (and SGMII) is
different from the PCS for 1000BASE-T in IEEE802.3.

The point I'm making here is that the capability of the link between
the MAC and the PHY doesn't define what the media link modes are. It
may define the speeds and duplexes that can be supported, and that
restricts the available link modes, but it doesn't define which
media "technologies" can be supported.

Hence, for example, the validate() implementation masking out
1000base-X but leaving 1000base-T on a *GMII link is pretty silly,
because whether one or the other is supported depends ultimately
what the *GMII link ends up being connected to.

> Consider the Xilinx 1G PCS. This PCS supports 1000BASE-X and SGMII, but
> only at full duplex. This caveat already rules out a completely
> bitmap-based solution (as phylink_get_linkmodes thinks that both of
> those interfaces are always full-duplex).

I don't see why you say this rules out a bitmap solution. You say that
it only supports full-duplex, and that is catered for in the current
solution: MAC_10 for example is actually MAC_10HD | MAC_10FD - which
allows one to specify that only MAC_10FD is supported and not MAC_10HD
in such a scenario.

Hmm. Also note that the validate() callback is not going away -
phylink_generic_validate() is a generic implementation of this that
gets rid of a lot of duplication and variability of implementation
that really shouldn't be there.

There are cases where the generic implementation will not be suitable,
and for this phylink_get_linkmodes() can be called directly, or I'd
even consider making phylink_caps_to_linkmodes() available if it is
useful. Or one can do it the "old way" that we currently have.

> There are also config options
> which (either as a feature or a consequence) disable SPEED_10 SGMII or
> autonegotiation (although I don't plan on supporting either of those).
> The "easiest" solution is simply to provide two callbacks like
> 
> 	void pcs_set_modes(struct phylink_pcs *pcs, ulong *supported,
> 			   phy_interface_t interface);
> 	bool pcs_mode_supported(struct phylink_pcs *pcs,
> 				phy_interface_t interface, int speed,
> 				int duplex);
> 
> perhaps with some generic substitutes. The former would need to be
> called from mac_validate, and the latter from mac_select_pcs/
> mac_prepare. This design is rather averse to genericization, so perhaps
> you have some suggestion?

I don't have a good answer for you at the moment - the PCS support
is something that has been recently added and is still quite young,
so these are the kinds of issues I'd expect to crop up.

> On the subject of PCS selection, mac_select_pcs should supply the whole
> state.

That may seem like a good thing to ask for, but not even phylink
knows what the full state is when calling the validation function,
nor when calling mac_select_pcs.

Let's take an example of the Marvell 88X3310 multi-speed PHY, which
supports 10G, 5G, 2.5G, 1G, 100M and 10M on copper, and 1G and 100M
on fiber, and can do all of that while connected to a single serdes
connection back to the MAC. As I've said above, it does this by
switching its MAC connection under its internal firmware between
10000Base-R, 5000Base-R, 2500Base-X, and SGMII. This PHY has been
found to be used in platforms, and discovered to also be in SFP
modules. Beyond programming the overall "host interface" mode, we
don't get a choice in which mode the PHY picks - that is determined
by the results of which interface comes up and autonegotiation on
that interface.

So, if the PHY decides to link on copper at 2500BASE-T, then we end
up with the MAC link operating at 2500BASE-X, and there's nothing
we can do about that.

The only way to restrict this is to know ahead of time what the
capabilities of the MAC and PCSes are, and to restrict the link
modes that phylib gives us in both the "supported" and "advertising"
fields, so the PHY will be programmed to e.g. not support 2500BASE-T
on copper if 2500BASE-X is not supported by the PCS, or 2.5G speeds
are not supported by the MAC.

This isn't something one can do when trying to bring the link up,
it's something that needs to be done when we are "putting the system
together" - in other words, when we are binding the PHY into the
link setup.

Now, this is quite horrible right now, because for PHYs like this,
phylink just asks the MAC's validate function "give me everything
you can support" when working this out - which won't be sufficient
going forward. With some of the changes you've prompted - making
more use of the supported_interfaces bitmap, and with further
adaption of phylib to also provide that information, we can start to
work out which interface modes the PHY _could_ select, and we can then
query the validate() function for what is possible for each of those
interface modes, and use that to bound the PHY capabilities. However,
at the moment, we just don't have that information available from
phylib.

> This is because the interface alone is insufficient to determine
> which PCS to select. For example, a PCS which supports full duplex but
> not half duplex should not be selected if the config specifies half
> duplex. Additionally, it should also support a selection of "no pcs".

Right now, "no pcs" is really not an option I'm afraid. The presence
of a PCS changes the phylink behaviour slightly . This is one of my
bug-bears. The evolution of phylink has meant that we need to keep
compatibility with how phylink used to work before we split the PCS
support - and we detect that by whether there is a PCS to determine
whether we need to operate with that compatibility. It probably was
a mistake to do that in hind sight.

If we can find a way to identify the old vs new drivers that doesn't
rely on the presence of a PCS, then we should be able to fix this to
allow the PCS to "vanish" in certain modes, but I do question whether
there would be any realistic implementations using it. If we have a
PHY connected to a serdes lane back to a set of PCS to support
different protocols on the serdes, then under what scenario would we
select "no pcs" - doesn't "no pcs" in that situation mean "we don't
know what protocol to drive the serdes link" ?

> Otherwise MACs which (optionally!) have PCSs will fail to configure. We
> should not fail when no PCS has yet been selected or when there is no
> PCS at all in some hardware configuration.  Further, why do we have this
> callback in the first place? Why don't we have drivers just do this in
> prepare()?

I added mac_select_pcs() because finding out that something isn't
supported in mac_prepare() is way too late - as explained above
where I talked about binding the PHY into the link setup. E.g. if
the "system" as a whole can't operate at 2.5G speeds, then we should
not allow the PHY to advertise 2500BASE-T. It is no good advertising
2500BASE-T, then having the PHY negotiate 2500BASE-T, select 2500BASE-X,
and then have mac_prepare() decide that can't be supported. The link
won't come up, and there's nothing that can be sensibly done. The
user sees the PHY indicating link, the link partner indicates link,
but the link is non-functional. That isn't a good user experience.

Whereas, if we know ahead of time that 2.5G can't be supported, we can
remove 2500BASE-T from the advertisement, and the PHY will instead
negotiate a slower speed - resulting in a working link, albiet slower.

I hope that explains why it is so important not to error out in
mac_prepare() because something wasn't properly handled in the
validate() step.

-- 
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