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

From: Ido Schimmel <[email-masked]>
Date: Sun, 7 Nov 2021 16:08:44 +0200

On Fri, Nov 05, 2021 at 09:53:31PM +0100, Maciej Machnikowski wrote:
> Add Documentation/networking/synce.rst describing new RTNL messages
> and respective NDO ops supporting SyncE (Synchronous Ethernet).
> 
> Signed-off-by: Maciej Machnikowski <[email-masked]>
> ---
>  Documentation/networking/synce.rst | 117 +++++++++++++++++++++++++++++
>  1 file changed, 117 insertions(+)
>  create mode 100644 Documentation/networking/synce.rst
> 
> diff --git a/Documentation/networking/synce.rst b/Documentation/networking/synce.rst
> new file mode 100644
> index 000000000000..4ca41fb9a481
> --- /dev/null
> +++ b/Documentation/networking/synce.rst
> @@ -0,0 +1,117 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +====================
> +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
> +and a dedicated TX clock input that is used as to transmit data to other nodes.
> +
> +The SyncE capable PHY is able to recover the incomning frequency of the data
> +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
> +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
> +filtering and holdover capabilities.
> +
> +The following interface can be applicable to diffferent packet network types
> +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.

Can you explain the difference between PHY outputs and EEC inputs? It is
no clear to me from the diagram.

How would the diagram look in a multi-port adapter where you have a
single EEC?

> +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

The first sentence seems to be redundant

> +
> +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
> +To support multiple recovered clock outputs from the same port, this message
> +will return the IFLA_RCLK_STATE_COUNT attribute containing the number of
> +active recovered clock outputs (N) and N IFLA_RCLK_STATE_OUT_IDX attributes
> +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.

Why do you need both RTM_GETRCLKRANGE and RTM_GETRCLKSTATE? Isn't
RTM_GETRCLKSTATE enough? Instead of skipping over "disabled" pins in the
range IFLA_RCLK_RANGE_MIN_PIN..IFLA_RCLK_RANGE_MAX_PIN, just report the
state (enabled / disable) for all

> +
> +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.

In the diagram you have two recovered clock outputs going into the EEC.
According to which the EEC is synchronized?

How does user space know which pins to enable?

> +
> +RTM_GETEECSTATE
> +----------------
> +Reads the state of the EEC or equivalent physical clock synchronizer.
> +This message returns the following attributes:
> +IFLA_EEC_STATE - current state of the EEC or equivalent clock generator.
> +		 The states returned in this attribute are aligned to the
> +		 ITU-T G.781 and are:
> +		  IF_EEC_STATE_INVALID - state is not valid
> +		  IF_EEC_STATE_FREERUN - clock is free-running
> +		  IF_EEC_STATE_LOCKED - clock is locked to the reference,
> +		                        but the holdover memory is not valid
> +		  IF_EEC_STATE_LOCKED_HO_ACQ - clock is locked to the reference
> +		                               and holdover memory is valid
> +		  IF_EEC_STATE_HOLDOVER - clock is in holdover mode
> +State is read from the netdev calling the:
> +int (*ndo_get_eec_state)(struct net_device *dev, enum if_eec_state *state,
> +			 u32 *src_idx, struct netlink_ext_ack *extack);
> +
> +IFLA_EEC_SRC_IDX - optional attribute returning the index of the reference that
> +		   is used for the current IFLA_EEC_STATE, i.e., the index of
> +		   the pin that the EEC is locked to.
> +
> +Will be returned only if the ndo_get_eec_src is implemented.
> \ No newline at end of file
> -- 
> 2.26.3
>