netdev - Re: [PATCH net-next 1/2] rtnetlink: Add new RTM_GETEECSTATE message to get SyncE status

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Message-ID: <20210908151852.7ad8a0f1@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Wed, 8 Sep 2021 15:18:52 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     "Machnikowski, Maciej" <maciej.machnikowski@...el.com>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "intel-wired-lan@...ts.osuosl.org" <intel-wired-lan@...ts.osuosl.org>,
        "richardcochran@...il.com" <richardcochran@...il.com>,
        "abyagowi@...com" <abyagowi@...com>,
        "Nguyen, Anthony L" <anthony.l.nguyen@...el.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        "Andrew Lunn" <andrew@...n.ch>, Michal Kubecek <mkubecek@...e.cz>,
        Saeed Mahameed <saeed@...nel.org>,
        Michael Chan <michael.chan@...adcom.com>
Subject: Re: [PATCH net-next 1/2] rtnetlink: Add new RTM_GETEECSTATE message
 to get SyncE status

On Wed, 8 Sep 2021 17:30:24 +0000 Machnikowski, Maciej wrote:
> Lane0
> ------------- |\  Pin0        RefN ____
> ------------- | |-----------------|     |      synced clk
>               | |-----------------| EEC |------------------
> ------------- |/ PinN         RefM|____ |
> Lane N      MUX
> 
> To get the full info a port needs to know the EEC state and which lane is used
> as a source (or rather - my lane or any other).

EEC here is what the PHY documentation calls "Cleanup PLL" right?

> The lane -> Pin mapping is buried in the PHY/MAC, but the source of frequency
> is in the EEC.

Not sure what "source of frequency" means here. There's a lot of
frequencies here.

> What's even more - the Pin->Ref mapping is board specific.

Breaking down the system into components we have:

Port
  A.1 Rx lanes
  A.2 Rx pins (outputs)
  A.3 Rx clk divider
  B.1 Tx lanes
  B.2 Tx pins (inputs)

ECC
  C.1 Inputs
  C.2 Outputs
  C.3 PLL state

In the most general case we want to be able to:
 map recovered clocks to PHY output pins (A.1 <> A.2)
 set freq div on the recovered clock (A.2 <> A.3)
 set the priorities of inputs on ECC (C.1)
 read the ECC state (C.3)
 control outputs of the ECC (C.2)
 select the clock source for port Tx (B.2 <> B.1)

As you said, pin -> ref mapping is board specific, so the API should
not assume knowledge of routing between Port and ECC. If it does just
give the pins matching names.

We don't have to implement the entire design but the pieces we do create
must be right for the larger context. With the current code the
ECC/Cleanup PLL is not represented as a separate entity, and mapping of
what source means is on the wrong "end" of the A.3 <> C.1 relationship.

> The viable solutions are:
> - Limit to the proposed "I drive the clock" vs "Someone drives it" and assume the
>    Driver returns all info
> - return the EEC Ref index, figure out which pin is connected to it and then check
>   which MAC/PHY lane that drives it.
> 
> I assume option one is easy to implement and keep in the future even if we
> finally move to option 2 once we define EEC/DPLL subsystem.
> 
> In future #1 can take the lock information from the DPLL subsystem, but
> will also enable simple deployments that won't expose the whole DPLL, 
> like a filter PLL embedded in a multiport PHY that will only work for
> SyncE in which case this API will only touch a single component.

Imagine a system with two cascaded switch ASICs and a bunch of PHYs.
How do you express that by pure extensions to the proposed API?
Here either the cleanup PLLs would be cascaded (subordinate one needs
to express that its "source" is another PLL) or single lane can be
designated as a source for both PLLs (but then there is only one
"source" bit and multiple "enum if_eec_state"s).

I think we can't avoid having a separate object for ECC/Cleanup PLL. 
You can add it as a subobject to devlink but new genetlink family seems
much preferable given the devlink instances themselves have unclear
semantics at this point. Or you can try to convince Richard that ECC
belongs as part of PTP :)

In fact I don't think you care about any of the PHY / port stuff
currently. All you need is the ECC side of the API. IIUC you have
relatively simple setup where there is only one pin per port, and
you don't care about syncing the Tx clock.