RE: [PATCH v4 net-next 2/4] ethtool: Add ability to configure recovered clock for SyncE feature

-----Original Message-----
From: Petr Machata <petrm@nvidia.com>
Sent: Monday, December 6, 2021 3:41 PM
To: Machnikowski, Maciej <redacted>
Subject: Re: [PATCH v4 net-next 2/4] ethtool: Add ability to configure
recovered clock for SyncE feature


Machnikowski, Maciej [off-list ref] writes:

quoted

quoted

-----Original Message-----
From: Petr Machata <petrm@nvidia.com>

Machnikowski, Maciej [off-list ref] writes:

quoted

Additionally, the EEC device may be instantiated by a totally
different driver, in which case the relation between its pins and
netdevs may not even be known.

Like an EEC, some PHYs, but the MAC driver does not know about both
pieces? Who sets up the connection between the two? The box admin
through some cabling? SoC designer?

Also, what does the external EEC actually do with the signal from the
PHY? Tune to it and forward to the other PHYs in the complex?

Yes - it can also apply HW filters to it.

Sounds like this device should have an EEC instance of its own then.

Maybe we need to call it something else than "EEC". PLL? Something that
does not have the standardization connotations, because several
instances would be present in a system with several NICs.

There will be no EEC/EEC subsystem, but the DPLL. Every driver would 
be able to create a DPLL object so that we can easily use it from 
non-netdev devices as well. See the other mail for more details.

quoted

The EEC model will not work when you have the following system:
SoC with some ethernet ports with driver A
Switch chip with N ports with driver B
EEC/DPLL with driver C
Both SoC and Switch ASIC can recover clock and use the cleaned
clock from the DPLL.

In that case you can't create any relation between EEC and recover
clock pins that would enable the EEC subsystem to control
recovered clocks, because you have 3 independent drivers.

I think that in that case you have several EEC instances. Those are
connected by some wiring that is external to the devices themselves. I
am not sure who should be in charge of describing the wiring. Device
tree? Config file?

In some complex systems you'll need to create a relation between netdevs
and DPLLs in a config file, so it is the only way to describe all possible
scenarios.
We can't assume any connections between them, as that's design specific,
just like PTP pins are. They have labels, but it's up to the system
integrator to define how they are used.
We can consider creating some of them if they are known to the driver
and belong to the same driver.

quoted

The model you proposed assumes that the MAC/Switch is in
charge of the DPLL, but that's not always true.

The EEC-centric model does not in fact assume that. It lets anyone to
set up an EEC object.

The netdev-centric UAPI assumes that the driver behind the netdev knows
about how many RCLK out pins there are. So it can certainly instantiate
a DPLL object instead, with those pins as external pins, and leave the
connection of the external pins to the EEC proper implicit.

Netdev will know how many RCLK outputs are there, as that's the function
of a given MAC/PHY/Retimer.

That gives userspace exactly the same information as the netdev-centric
UAPI, but now userspace doesn't need to know about netdevs, and
synchronously-spinning drives, and GPS receivers, each of which is
handled through a dedicated set of netlink messages / sysctls / what
have you. The userspace needs to know about EEC subsystem, and that's
it.

I believe the direction is to make the connection between a netdev and
its related DPLL that's serving as EEC in a similar way the link to a PTP 
device is created. Userspace app will need to go to DPLL subsystem
to understand what's the current frequency source for a given netdev.

That's still independent uAPI from the one defined by those patches.

quoted

The model where recovered clock outputs are controlled independently
can support both models and is more flexible. It can also address the

- Anyone can instantiate EEC objects
- Only things with ports instantiate netdevs

How is the latter one more flexible?

- Everything can instantiate DPLL object,
- Only a netdev can instantiate recovered clock outputs, which can be 
  connected to any other part of the system - not only a DPLL.