Date:   Fri, 3 Dec 2021 15:26:41 +0100
From:   Petr Machata <petrm@...dia.com>
To:     "Machnikowski, Maciej" <maciej.machnikowski@...el.com>
CC:     Ido Schimmel <idosch@...sch.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "intel-wired-lan@...ts.osuosl.org" <intel-wired-lan@...ts.osuosl.org>,
        "Kubalewski, Arkadiusz" <arkadiusz.kubalewski@...el.com>,
        "richardcochran@...il.com" <richardcochran@...il.com>,
        "abyagowi@...com" <abyagowi@...com>,
        "Nguyen, Anthony L" <anthony.l.nguyen@...el.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        "mkubecek@...e.cz" <mkubecek@...e.cz>,
        "saeed@...nel.org" <saeed@...nel.org>,
        "michael.chan@...adcom.com" <michael.chan@...adcom.com>,
        "petrm@...dia.com" <petrm@...dia.com>
Subject: Re: [PATCH v4 net-next 2/4] ethtool: Add ability to configure
 recovered clock for SyncE feature


Machnikowski, Maciej <maciej.machnikowski@...el.com> writes:

>> -----Original Message-----
>> From: Ido Schimmel <idosch@...sch.org>
>>
>> On Thu, Dec 02, 2021 at 03:17:06PM +0000, Machnikowski, Maciej wrote:
>> > > -----Original Message-----
>> > > From: Ido Schimmel <idosch@...sch.org>
>> > >
>> > > On Wed, Dec 01, 2021 at 07:02:06PM +0100, Maciej Machnikowski wrote:
>> > > Looking at the diagram from the previous submission [1]:
>> > >
>> > >       ┌──────────┬──────────┐
>> > >       │ RX       │ TX       │
>> > >   1   │ ports    │ ports    │ 1
>> > >   ───►├─────┐    │          ├─────►
>> > >   2   │     │    │          │ 2
>> > >   ───►├───┐ │    │          ├─────►
>> > >   3   │   │ │    │          │ 3
>> > >   ───►├─┐ │ │    │          ├─────►
>> > >       │ ▼ ▼ ▼    │          │
>> > >       │ ──────   │          │
>> > >       │ \____/   │          │
>> > >       └──┼──┼────┴──────────┘
>> > >         1│ 2│        ▲
>> > >  RCLK out│  │        │ TX CLK in
>> > >          ▼  ▼        │
>> > >        ┌─────────────┴───┐
>> > >        │                 │
>> > >        │       SEC       │
>> > >        │                 │
>> > >        └─────────────────┘
>> > >
>> > > Given a netdev (1, 2 or 3 in the diagram), the RCLK_SET message allows
>> > > me to redirect the frequency recovered from this netdev to the EEC via
>> > > either pin 1, pin 2 or both.
>> > >
>> > > Given a netdev, the RCLK_GET message allows me to query the range of
>> > > pins (RCLK out 1-2 in the diagram) through which the frequency can be
>> > > fed into the EEC.
>> > >
>> > > Questions:
>> > >
>> > > 1. The query for all the above netdevs will return the same range
>> > > of pins. How does user space know that these are the same pins?
>> > > That is, how does user space know that RCLK_SET message to
>> > > redirect the frequency recovered from netdev 1 to pin 1 will be
>> > > overridden by the same message but for netdev 2?
>> >
>> > We don't have a way to do so right now. When we have EEC subsystem
>> > in place the right thing to do will be to add EEC input index and
>> > EEC index as additional arguments
>> >
>> > > 2. How does user space know the mapping between a netdev and an
>> > > EEC? That is, how does user space know that RCLK_SET message for
>> > > netdev 1 will cause the Tx frequency of netdev 2 to change
>> > > according to the frequency recovered from netdev 1?
>> >
>> > Ditto - currently we don't have any entity to link the pins to ATM,
>> > but we can address that in userspace just like PTP pins are used
>> > now
>> >
>> > > 3. If user space sends two RCLK_SET messages to redirect the
>> > > frequency recovered from netdev 1 to RCLK out 1 and from netdev 2
>> > > to RCLK out 2, how does it know which recovered frequency is
>> > > actually used an input to the EEC?
>> 
>> User space doesn't know this as well?
>
> In current model it can come from the config file. Once we implement DPLL
> subsystem we can implement connection between pins and DPLLs if they are
> known.
>
>> > >
>> > > 4. Why these pins are represented as attributes of a netdev and not as
>> > > attributes of the EEC? That is, why are they represented as output pins
>> > > of the PHY as opposed to input pins of the EEC?
>> >
>> > They are 2 separate beings. Recovered clock outputs are controlled
>> > separately from EEC inputs.
>> 
>> Separate how? What does it mean that they are controlled separately? In
>> which sense? That redirection of recovered frequency to pin is
>> controlled via PHY registers whereas priority setting between EEC inputs
>> is controlled via EEC registers? If so, this is an implementation detail
>> of a specific design. It is not of any importance to user space.
>
> They belong to different devices. EEC registers are physically in the DPLL
> hanging over I2C and recovered clocks are in the PHY/integrated PHY in
> the MAC. Depending on system architecture you may have control over
> one piece only

What does ETHTOOL_MSG_RCLK_SET actually configure, physically? Say I
have this message:

ETHTOOL_MSG_RCLK_SET dev = eth0
- ETHTOOL_A_RCLK_OUT_PIN_IDX = n
- ETHTOOL_A_RCLK_PIN_FLAGS |= ETHTOOL_RCLK_PIN_FLAGS_ENA

Eventually this lands in ops->set_rclk_out(dev, out_idx, new_state).
What does the MAC driver do next?

>> > If we mix them it'll be hard to control everything especially that a
>> > single EEC can support multiple devices.
>> 
>> Hard how? Please provide concrete examples.
>
> From the EEC perspective it's one to many relation - one EEC input pin will serve
> even 4,16,48 netdevs. I don't see easy way of starting from EEC input of EEC device
> and figuring out which netdevs are connected to it to talk to the right one.
> In current model it's as simple as:
> - I received QL-PRC on netdev ens4f0
> - I send back enable recovered clock on pin 0 of the ens4f0

How do I know it's pin 0 though? Config file?

> - go to EEC that will be linked to it
> - see the state of it - if its locked - report QL-EEC downsteam
>
> How would you this control look in the EEC/DPLL implementation? Maybe
> I missed something.

In the EEC-centric model this is what happens:

- QL-PRC packet is received on ens4f0
- Userspace consults a UAPI to figure out what EEC and pin ID this
  netdevice corresponds to
- Userspace instructs through a UAPI the indicated EEC to use the
  indicated pin as a source
- Userspace then monitors the indicated EEC through a UAPI. When the EEC
  locks, QL-EEC is reported downstream

>> What do you mean by "multiple devices"? A multi-port adapter with a
>> single EEC or something else?
>
> Multiple MACs that use a single EEC clock.
>
>> > Also if we make those pins attributes of the EEC it'll become extremally hard
>> > to map them to netdevs and control them from the userspace app that will
>> > receive the ESMC message with a given QL level on netdev X.
>> 
>> Hard how? What is the problem with something like:
>> 
>> # eec set source 1 type netdev dev swp1
>> 
>> The EEC object should be registered by the same entity that registers
>> the netdevs whose Tx frequency is controlled by the EEC, the MAC driver.
>
> But the EEC object may not be controlled by the MAC - in which case
> this model won't work.

In that case the driver for the device that controls EEC would
instantiates the object. It doesn't have to be a MAC driver.

But if it is controlled by the MAC, the MAC driver instantiates it. And
can set up the connection between the MAC and the EEC, so that in the
shell snippet above "eec" knows how to get the EEC handle from the
netdevice.

>> >
>> > > 5. What is the problem with the following model?
>> > >
>> > > - The EEC is a separate object with following attributes:
>> > >   * State: Invalid / Freerun / Locked / etc
>> > >   * Sources: Netdev / external / etc
>> > >   * Potentially more
>> > >
>> > > - Notifications are emitted to user space when the state of the EEC
>> > >   changes. Drivers will either poll the state from the device or get
>> > >   interrupts
>> > >
>> > > - The mapping from netdev to EEC is queried via ethtool
>> >
>> > Yep - that will be part of the EEC (DPLL) subsystem
>>
>> This model avoids all the problems I pointed out in the current
>> proposal.
>
> That's the go-to model, but first we need control over the source as
> well :)

Why is that? Can you illustrate a case that breaks with the above model?

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