The Linux kernel dpll subsystem


PLL - Phase Locked Loop is an electronic circuit which syntonizes clock signal of a device with an external clock signal. Effectively enabling device to run on the same clock signal beat as provided on a PLL input.

DPLL - Digital Phase Locked Loop is an integrated circuit which in addition to plain PLL behavior incorporates a digital phase detector and may have digital divider in the loop. As a result, the frequency on DPLL's input and output may be configurable.


The main purpose of dpll subsystem is to provide general interface to configure devices that use any kind of Digital PLL and could use different sources of input signal to synchronize to, as well as different types of outputs. The main interface is NETLINK_GENERIC based protocol with an event monitoring multicast group defined.

Device object

Single dpll device object means single Digital PLL circuit and bunch of connected pins. It reports the supported modes of operation and current status to the user in response to the do request of netlink command DPLL_CMD_DEVICE_GET and list of dplls registered in the subsystem with dump netlink request of the same command. Changing the configuration of dpll device is done with do request of netlink DPLL_CMD_DEVICE_SET command. A device handle is DPLL_A_ID, it shall be provided to get or set configuration of particular device in the system. It can be obtained with a DPLL_CMD_DEVICE_GET dump request or a DPLL_CMD_DEVICE_ID_GET do request, where the one must provide attributes that result in single device match.

Pin object

A pin is amorphic object which represents either input or output, it could be internal component of the device, as well as externally connected. The number of pins per dpll vary, but usually multiple pins shall be provided for a single dpll device. Pin's properties, capabilities and status is provided to the user in response to do request of netlink DPLL_CMD_PIN_GET command. It is also possible to list all the pins that were registered in the system with dump request of DPLL_CMD_PIN_GET command. Configuration of a pin can be changed by do request of netlink DPLL_CMD_PIN_SET command. Pin handle is a DPLL_A_PIN_ID, it shall be provided to get or set configuration of particular pin in the system. It can be obtained with DPLL_CMD_PIN_GET dump request or DPLL_CMD_PIN_ID_GET do request, where user provides attributes that result in single pin match.

Pin selection

In general, selected pin (the one which signal is driving the dpll device) can be obtained from DPLL_A_PIN_STATE attribute, and only one pin shall be in DPLL_PIN_STATE_CONNECTED state for any dpll device.

Pin selection can be done either manually or automatically, depending on hardware capabilities and active dpll device work mode (DPLL_A_MODE attribute). The consequence is that there are differences for each mode in terms of available pin states, as well as for the states the user can request for a dpll device.

In manual mode (DPLL_MODE_MANUAL) the user can request or receive one of following pin states:

  • DPLL_PIN_STATE_CONNECTED - the pin is used to drive dpll device

  • DPLL_PIN_STATE_DISCONNECTED - the pin is not used to drive dpll device

In automatic mode (DPLL_MODE_AUTOMATIC) the user can request or receive one of following pin states:

  • DPLL_PIN_STATE_SELECTABLE - the pin shall be considered as valid input for automatic selection algorithm

  • DPLL_PIN_STATE_DISCONNECTED - the pin shall be not considered as a valid input for automatic selection algorithm

In automatic mode (DPLL_MODE_AUTOMATIC) the user can only receive pin state DPLL_PIN_STATE_CONNECTED once automatic selection algorithm locks a dpll device with one of the inputs.

Shared pins

A single pin object can be attached to multiple dpll devices. Then there are two groups of configuration knobs:

  1. Set on a pin - the configuration affects all dpll devices pin is registered to (i.e., DPLL_A_PIN_FREQUENCY),

  2. Set on a pin-dpll tuple - the configuration affects only selected dpll device (i.e., DPLL_A_PIN_PRIO, DPLL_A_PIN_STATE, DPLL_A_PIN_DIRECTION).

MUX-type pins

A pin can be MUX-type, it aggregates child pins and serves as a pin multiplexer. One or more pins are registered with MUX-type instead of being directly registered to a dpll device. Pins registered with a MUX-type pin provide user with additional nested attribute DPLL_A_PIN_PARENT_PIN for each parent they were registered with. If a pin was registered with multiple parent pins, they behave like a multiple output multiplexer. In this case output of a DPLL_CMD_PIN_GET would contain multiple pin-parent nested attributes with current state related to each parent, like:

'pin': [{{
  'clock-id': 282574471561216,
  'module-name': 'ice',
  'capabilities': 4,
  'id': 13,
  'parent-pin': [
  {'parent-id': 2, 'state': 'connected'},
  {'parent-id': 3, 'state': 'disconnected'}
  'type': 'synce-eth-port'

Only one child pin can provide its signal to the parent MUX-type pin at a time, the selection is done by requesting change of a child pin state on desired parent, with the use of DPLL_A_PIN_PARENT nested attribute. Example of netlink set state on parent pin message format:


child pin id


nested attribute for requesting configuration related to parent pin


parent pin id


requested pin state on parent

Pin priority

Some devices might offer a capability of automatic pin selection mode (enum value DPLL_MODE_AUTOMATIC of DPLL_A_MODE attribute). Usually, automatic selection is performed on the hardware level, which means only pins directly connected to the dpll can be used for automatic input pin selection. In automatic selection mode, the user cannot manually select a input pin for the device, instead the user shall provide all directly connected pins with a priority DPLL_A_PIN_PRIO, the device would pick a highest priority valid signal and use it to control the DPLL device. Example of netlink set priority on parent pin message format:


configured pin id


nested attribute for requesting configuration related to parent dpll device


parent dpll device id


requested pin prio on parent dpll

Child pin of MUX-type pin is not capable of automatic input pin selection, in order to configure active input of a MUX-type pin, the user needs to request desired pin state of the child pin on the parent pin, as described in the MUX-type pins chapter.

Phase offset measurement and adjustment

Device may provide ability to measure a phase difference between signals on a pin and its parent dpll device. If pin-dpll phase offset measurement is supported, it shall be provided with DPLL_A_PIN_PHASE_OFFSET attribute for each parent dpll device.

Device may also provide ability to adjust a signal phase on a pin. If pin phase adjustment is supported, minimal and maximal values that pin handle shall be provide to the user on DPLL_CMD_PIN_GET respond with DPLL_A_PIN_PHASE_ADJUST_MIN and DPLL_A_PIN_PHASE_ADJUST_MAX attributes. Configured phase adjust value is provided with DPLL_A_PIN_PHASE_ADJUST attribute of a pin, and value change can be requested with the same attribute with DPLL_CMD_PIN_SET command.


configured pin id


attr minimum value of phase adjustment


attr maximum value of phase adjustment


attr configured value of phase adjustment on parent dpll device


nested attribute for requesting configuration on given parent dpll device


parent dpll device id


attr measured phase difference between a pin and parent dpll device

All phase related values are provided in pico seconds, which represents time difference between signals phase. The negative value means that phase of signal on pin is earlier in time than dpll's signal. Positive value means that phase of signal on pin is later in time than signal of a dpll.

Phase adjust (also min and max) values are integers, but measured phase offset values are fractional with 3-digit decimal places and shell be divided with DPLL_PIN_PHASE_OFFSET_DIVIDER to get integer part and modulo divided to get fractional part.

Configuration commands group

Configuration commands are used to get information about registered dpll devices (and pins), as well as set configuration of device or pins. As dpll devices must be abstracted and reflect real hardware, there is no way to add new dpll device via netlink from user space and each device should be registered by its driver.

All netlink commands require GENL_ADMIN_PERM. This is to prevent any spamming/DoS from unauthorized userspace applications.

SET commands format

DPLL_CMD_DEVICE_SET - to target a dpll device, the user provides DPLL_A_ID, which is unique identifier of dpll device in the system, as well as parameter being configured (DPLL_A_MODE).

DPLL_CMD_PIN_SET - to target a pin user must provide a DPLL_A_PIN_ID, which is unique identifier of a pin in the system. Also configured pin parameters must be added. If DPLL_A_PIN_FREQUENCY is configured, this affects all the dpll devices that are connected with the pin, that is why frequency attribute shall not be enclosed in DPLL_A_PIN_PARENT_DEVICE. Other attributes: DPLL_A_PIN_PRIO, DPLL_A_PIN_STATE or DPLL_A_PIN_DIRECTION must be enclosed in DPLL_A_PIN_PARENT_DEVICE as their configuration relates to only one of parent dplls, targeted by DPLL_A_PIN_PARENT_ID attribute which is also required inside that nest. For MUX-type pins the DPLL_A_PIN_STATE attribute is configured in similar way, by enclosing required state in DPLL_A_PIN_PARENT_PIN nested attribute and targeted parent pin id in DPLL_A_PIN_PARENT_ID.

In general, it is possible to configure multiple parameters at once, but internally each parameter change will be invoked separately, where order of configuration is not guaranteed by any means.

Configuration pre-defined enums

enum dpll_mode

working modes a dpll can support, differentiates if and how dpll selects one of its inputs to syntonize with it, valid values for DPLL_A_MODE attribute



input can be only selected by sending a request to dpll


highest prio input pin auto selected by dpll

enum dpll_lock_status

provides information of dpll device lock status, valid values for DPLL_A_LOCK_STATUS attribute



dpll was not yet locked to any valid input (or forced by setting DPLL_A_MODE to DPLL_MODE_DETACHED)


dpll is locked to a valid signal, but no holdover available


dpll is locked and holdover acquired


dpll is in holdover state - lost a valid lock or was forced by disconnecting all the pins (latter possible only when dpll lock-state was already DPLL_LOCK_STATUS_LOCKED_HO_ACQ, if dpll lock-state was not DPLL_LOCK_STATUS_LOCKED_HO_ACQ, the dpll's lock-state shall remain DPLL_LOCK_STATUS_UNLOCKED)

enum dpll_type

type of dpll, valid values for DPLL_A_TYPE attribute



dpll produces Pulse-Per-Second signal


dpll drives the Ethernet Equipment Clock

enum dpll_pin_type

defines possible types of a pin, valid values for DPLL_A_PIN_TYPE attribute



aggregates another layer of selectable pins


external input


ethernet port PHY's recovered clock


device internal oscillator


GNSS recovered clock

enum dpll_pin_direction

defines possible direction of a pin, valid values for DPLL_A_PIN_DIRECTION attribute



pin used as a input of a signal


pin used to output the signal

enum dpll_pin_state

defines possible states of a pin, valid values for DPLL_A_PIN_STATE attribute



pin connected, active input of phase locked loop


pin disconnected, not considered as a valid input


pin enabled for automatic input selection

enum dpll_pin_capabilities

defines possible capabilities of a pin, valid flags on DPLL_A_PIN_CAPABILITIES attribute



pin direction can be changed


pin priority can be changed


pin state can be changed


dpll device can provide notifications regarding status changes of the device, i.e. lock status changes, input/output changes or other alarms. There is one multicast group that is used to notify user-space apps via netlink socket: DPLL_MCGRP_MONITOR

Notifications messages:


dpll device was created


dpll device was deleted


dpll device has changed


dpll pin was created


dpll pin was deleted


dpll pin has changed

Events format is the same as for the corresponding get command. Format of DPLL_CMD_DEVICE_ events is the same as response of DPLL_CMD_DEVICE_GET. Format of DPLL_CMD_PIN_ events is same as response of DPLL_CMD_PIN_GET.

Device driver implementation

Device is allocated by dpll_device_get() call. Second call with the same arguments will not create new object but provides pointer to previously created device for given arguments, it also increases refcount of that object. Device is deallocated by dpll_device_put() call, which first decreases the refcount, once refcount is cleared the object is destroyed.

Device should implement set of operations and register device via dpll_device_register() at which point it becomes available to the users. Multiple driver instances can obtain reference to it with dpll_device_get(), as well as register dpll device with their own ops and priv.

The pins are allocated separately with dpll_pin_get(), it works similarly to dpll_device_get(). Function first creates object and then for each call with the same arguments only the object refcount increases. Also dpll_pin_put() works similarly to dpll_device_put().

A pin can be registered with parent dpll device or parent pin, depending on hardware needs. Each registration requires registerer to provide set of pin callbacks, and private data pointer for calling them:

  • dpll_pin_register() - register pin with a dpll device,

  • dpll_pin_on_pin_register() - register pin with another MUX type pin.

Notifications of adding or removing dpll devices are created within subsystem itself. Notifications about registering/deregistering pins are also invoked by the subsystem. Notifications about status changes either of dpll device or a pin are invoked in two ways:

  • after successful change was requested on dpll subsystem, the subsystem calls corresponding notification,

  • requested by device driver with dpll_device_change_ntf() or dpll_pin_change_ntf() when driver informs about the status change.

The device driver using dpll interface is not required to implement all the callback operation. Nevertheless, there are few required to be implemented. Required dpll device level callback operations:

  • .mode_get,

  • .lock_status_get.

Required pin level callback operations:

  • .state_on_dpll_get (pins registered with dpll device),

  • .state_on_pin_get (pins registered with parent pin),

  • .direction_get.

Every other operation handler is checked for existence and -EOPNOTSUPP is returned in case of absence of specific handler.

The simplest implementation is in the OCP TimeCard driver. The ops structures are defined like this:

static const struct dpll_device_ops dpll_ops = {
        .lock_status_get = ptp_ocp_dpll_lock_status_get,
        .mode_get = ptp_ocp_dpll_mode_get,
        .mode_supported = ptp_ocp_dpll_mode_supported,

static const struct dpll_pin_ops dpll_pins_ops = {
        .frequency_get = ptp_ocp_dpll_frequency_get,
        .frequency_set = ptp_ocp_dpll_frequency_set,
        .direction_get = ptp_ocp_dpll_direction_get,
        .direction_set = ptp_ocp_dpll_direction_set,
        .state_on_dpll_get = ptp_ocp_dpll_state_get,

The registration part is then looks like this part:

clkid = pci_get_dsn(pdev);
bp->dpll = dpll_device_get(clkid, 0, THIS_MODULE);
if (IS_ERR(bp->dpll)) {
        err = PTR_ERR(bp->dpll);
        dev_err(&pdev->dev, "dpll_device_alloc failed\n");
        goto out;

err = dpll_device_register(bp->dpll, DPLL_TYPE_PPS, &dpll_ops, bp);
if (err)
        goto out;

for (i = 0; i < OCP_SMA_NUM; i++) {
        bp->sma[i].dpll_pin = dpll_pin_get(clkid, i, THIS_MODULE, &bp->sma[i].dpll_prop);
        if (IS_ERR(bp->sma[i].dpll_pin)) {
                err = PTR_ERR(bp->dpll);
                goto out_dpll;

        err = dpll_pin_register(bp->dpll, bp->sma[i].dpll_pin, &dpll_pins_ops,
        if (err) {
                goto out_dpll;

In the error path we have to rewind every allocation in the reverse order:

while (i) {
        dpll_pin_unregister(bp->dpll, bp->sma[i].dpll_pin, &dpll_pins_ops, &bp->sma[i]);

More complex example can be found in Intel's ICE driver or nVidia's mlx5 driver.

SyncE enablement

For SyncE enablement it is required to allow control over dpll device for a software application which monitors and configures the inputs of dpll device in response to current state of a dpll device and its inputs. In such scenario, dpll device input signal shall be also configurable to drive dpll with signal recovered from the PHY netdevice. This is done by exposing a pin to the netdevice - attaching pin to the netdevice itself with netdev_dpll_pin_set(struct net_device *dev, struct dpll_pin *dpll_pin). Exposed pin id handle DPLL_A_PIN_ID is then identifiable by the user as it is attached to rtnetlink respond to get RTM_NEWLINK command in nested attribute IFLA_DPLL_PIN.