Re: [PATCH v2 net-next 6/6] docs: net: Add description of SyncE interfaces

Machnikowski, Maciej [off-list ref] writes:

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Maciej Machnikowski [off-list ref] writes:

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+====================
+Synchronous Ethernet
+====================
+
+Synchronous Ethernet networks use a physical layer clock to syntonize
+the frequency across different network elements.
+
+Basic SyncE node defined in the ITU-T G.8264 consist of an Ethernet
+Equipment Clock (EEC) and a PHY that has dedicated outputs of recovered

clocks

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+and a dedicated TX clock input that is used as to transmit data to other

nodes.

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+
+The SyncE capable PHY is able to recover the incomning frequency of the

data

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+stream on RX lanes and redirect it (sometimes dividing it) to recovered
+clock outputs. In SyncE PHY the TX frequency is directly dependent on the
+input frequency - either on the PHY CLK input, or on a dedicated
+TX clock input.
+
+      ┌───────────┬──────────┐
+      │ RX        │ TX       │
+  1   │ lanes     │ lanes    │ 1
+  ───►├──────┐    │          ├─────►
+  2   │      │    │          │ 2
+  ───►├──┐   │    │          ├─────►
+  3   │  │   │    │          │ 3
+  ───►├─▼▼   ▼    │          ├─────►
+      │ ──────    │          │
+      │ \____/    │          │
+      └──┼──┼─────┴──────────┘
+        1│ 2│        ▲
+ RCLK out│  │        │ TX CLK in
+         ▼  ▼        │
+       ┌─────────────┴───┐
+       │                 │
+       │       EEC       │
+       │                 │
+       └─────────────────┘
+
+The EEC can synchronize its frequency to one of the synchronization

inputs

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+either clocks recovered on traffic interfaces or (in advanced deployments)
+external frequency sources.
+
+Some EEC implementations can select synchronization source through
+priority tables and synchronization status messaging and provide

necessary

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+filtering and holdover capabilities.
+
+The following interface can be applicable to diffferent packet network

types

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+following ITU-T G.8261/G.8262 recommendations.
+
+Interface
+=========
+
+The following RTNL messages are used to read/configure SyncE recovered
+clocks.
+
+RTM_GETRCLKRANGE
+-----------------
+Reads the allowed pin index range for the recovered clock outputs.
+This can be aligned to PHY outputs or to EEC inputs, whichever is
+better for a given application.
+Will call the ndo_get_rclk_range function to read the allowed range
+of output pin indexes.
+Will call ndo_get_rclk_range to determine the allowed recovered clock
+range and return them in the IFLA_RCLK_RANGE_MIN_PIN and the
+IFLA_RCLK_RANGE_MAX_PIN attributes
+
+RTM_GETRCLKSTATE
+-----------------
+Read the state of recovered pins that output recovered clock from
+a given port. The message will contain the number of assigned clocks
+(IFLA_RCLK_STATE_COUNT) and an N pin indexes in

IFLA_RCLK_STATE_OUT_IDX

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+To support multiple recovered clock outputs from the same port, this

message

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+will return the IFLA_RCLK_STATE_COUNT attribute containing the number

of

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+active recovered clock outputs (N) and N IFLA_RCLK_STATE_OUT_IDX

attributes

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+listing the active output indexes.
+This message will call the ndo_get_rclk_range to determine the allowed
+recovered clock indexes and then will loop through them, calling
+the ndo_get_rclk_state for each of them.

Let me make sure I understand the model that you propose. Specifically
from the point of view of a multi-port device, because that's my
immediate use case.

RTM_GETRCLKRANGE would report number of "pins" that matches the
number
of lanes in the system. So e.g. a 32-port switch, where each port has 4
lanes, would give a range of [1; 128], inclusive. (Or maybe [0; 128) or
whatever.)

RTM_GETRCLKSTATE would then return some subset of those pins,
depending
on which lanes actually managed to establish a connection and carry a
valid clock signal. So, say, [1, 2, 3, 4] if the first port has e.g. a
100Gbps established.

Those 2 will be merged into a single RTM_GETRCLKSTATE that will report
the state of all available pins for a given port.

Also lanes here should really be ports - will fix in next revision.

But the logic will be: 
Call the RTM_GETRCLKSTATE. It will return the list of pins and their state
for a given port. Once you read the range you will send the RTM_SETRCLKSTATE
to enable the redirection to a given RCLK output from the PHY. If your DPLL/EEC
is configured to accept it automatically - it's all you need to do and you need to
wait for the right state of the EEC (locked/locked with HO).

Ha, ok, so the RANGE call goes away, it's all in the RTM_GETRCLKSTATE.

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+
+RTM_SETRCLKSTATE
+-----------------
+Sets the redirection of the recovered clock for a given pin. This message
+expects one attribute:
+struct if_set_rclk_msg {
+	__u32 ifindex; /* interface index */
+	__u32 out_idx; /* output index (from a valid range)
+	__u32 flags; /* configuration flags */
+};
+
+Supported flags are:
+SET_RCLK_FLAGS_ENA - if set in flags - the given output will be enabled,
+		     if clear - the output will be disabled.

OK, so here I set up the tracking. ifindex tells me which EEC to
configure, out_idx is the pin to track, flags tell me whether to set up
the tracking or tear it down. Thus e.g. on port 2, track pin 2, because
I somehow know that lane 2 has the best clock.

It's bound to ifindex to know which PHY port you interact with. It has nothing to
do with the EEC yet.

It has in the sense that I'm configuring "TX CLK in", which leads from
EEC to the port.

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If the above is broadly correct, I've got some questions.

First, what if more than one out_idx is set? What are drivers / HW meant
to do with this? What is the expected behavior?

Expected behavior is deployment specific. You can use different phy recovered
clock outputs to implement active/passive mode of clock failover.

How? Which one is primary and which one is backup? I just have two
enabled pins...

Wouldn't failover be implementable in a userspace daemon? That would get
a notification from the system that holdover was entered, and can
reconfigure tracking to another pin based on arbitrary rules.

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Also GETRCLKSTATE and SETRCLKSTATE have a somewhat different scope:
one
reports which pins carry a clock signal, the other influences tracking.
That seems wrong. There also does not seems to be an UAPI to retrieve
the tracking settings.

They don't. Get reads the redirection state and SET sets it - nothing more,
nothing less. In ICE we use EEC pin indexes so that the model translates easier
to the one when we support DPLL subsystem.

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Second, as a user-space client, how do I know that if ports 1 and 2 both
report pin range [A; B], that they both actually share the same
underlying EEC? Is there some sort of coordination among the drivers,
such that each pin in the system has a unique ID?

For now we don't, as we don't have EEC subsystem. But that can be solved
by a config file temporarily.

I think it would be better to model this properly from day one.

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Further, how do I actually know the mapping from ports to pins? E.g. as
a user, I might know my master is behind swp1. How do I know what pins
correspond to that port? As a user-space tool author, how do I help
users to do something like "eec set clock eec0 track swp1"?

That's why driver needs to be smart there and return indexes properly.

What do you mean, properly? Up there you have RTM_GETRCLKRANGE that just
gives me a min and a max. Is there a policy about how to correlate
numbers in that range to... ifindices, netdevice names, devlink port
numbers, I don't know, something?

How do several drivers coordinate this numbering among themselves? Is
there a core kernel authority that manages pin number de/allocations?

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Additionally, how would things like external GPSs or 1pps be modeled? I
guess the driver would know about such interface, and would expose it as
a "pin". When the GPS signal locks, the driver starts reporting the pin
in the RCLK set. Then it is possible to set up tracking of that pin.

That won't be enabled before we get the DPLL subsystem ready.

It might prove challenging to retrofit an existing netdev-centric
interface into a more generic model. It would be better to model this
properly from day one, and OK, if we can carve out a subset of that
model to implement now, and leave the rest for later, fine. But the
current model does not strike me as having a natural migration path to
something more generic. E.g. reporting the EEC state through the
interfaces attached to that EEC... like, that will have to stay, even at
a time when it is superseded by a better interface.

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It seems to me it would be easier to understand, and to write user-space
tools and drivers for, a model that has EEC as an explicit first-class
object. That's where the EEC state naturally belongs, that's where the
pin range naturally belongs. Netdevs should have a reference to EEC and
pins, not present this information as if they own it. A first-class EEC
would also allow to later figure out how to hook up PHC and EEC.

We have the userspace tool, but can’t upstream it until we define
kernel Interfaces. It's paragraph 22 :(

I'm sure you do, presumably you test this somehow. Still, as a potential
consumer of that interface, I will absolutely poke at it to figure out
how to use it, what it lets me to do, and what won't work.

BTW, what we've done in the past in a situation like this was, here's
the current submission, here's a pointer to a GIT with more stuff we
plan to send later on, here's a pointer to a GIT with the userspace
stuff. I doubt anybody actually looks at that code, ain't nobody got
time for that, but really there's no catch 22.