[linux-2.6-block.git] / Documentation / driver-api / ptp.rst

.. SPDX-License-Identifier: GPL-2.0

===========================================
PTP hardware clock infrastructure for Linux
===========================================

  This patch set introduces support for IEEE 1588 PTP clocks in
  Linux. Together with the SO_TIMESTAMPING socket options, this
  presents a standardized method for developing PTP user space
  programs, synchronizing Linux with external clocks, and using the
  ancillary features of PTP hardware clocks.

  A new class driver exports a kernel interface for specific clock
  drivers and a user space interface. The infrastructure supports a
  complete set of PTP hardware clock functionality.

  + Basic clock operations
    - Set time
    - Get time
    - Shift the clock by a given offset atomically
    - Adjust clock frequency

  + Ancillary clock features
    - Time stamp external events
    - Period output signals configurable from user space
    - Synchronization of the Linux system time via the PPS subsystem

PTP hardware clock kernel API
=============================

   A PTP clock driver registers itself with the class driver. The
   class driver handles all of the dealings with user space. The
   author of a clock driver need only implement the details of
   programming the clock hardware. The clock driver notifies the class
   driver of asynchronous events (alarms and external time stamps) via
   a simple message passing interface.

   The class driver supports multiple PTP clock drivers. In normal use
   cases, only one PTP clock is needed. However, for testing and
   development, it can be useful to have more than one clock in a
   single system, in order to allow performance comparisons.

PTP hardware clock user space API
=================================

   The class driver also creates a character device for each
   registered clock. User space can use an open file descriptor from
   the character device as a POSIX clock id and may call
   clock_gettime, clock_settime, and clock_adjtime.  These calls
   implement the basic clock operations.

   User space programs may control the clock using standardized
   ioctls. A program may query, enable, configure, and disable the
   ancillary clock features. User space can receive time stamped
   events via blocking read() and poll().

Writing clock drivers
=====================

   Clock drivers include include/linux/ptp_clock_kernel.h and register
   themselves by presenting a 'struct ptp_clock_info' to the
   registration method. Clock drivers must implement all of the
   functions in the interface. If a clock does not offer a particular
   ancillary feature, then the driver should just return -EOPNOTSUPP
   from those functions.

   Drivers must ensure that all of the methods in interface are
   reentrant. Since most hardware implementations treat the time value
   as a 64 bit integer accessed as two 32 bit registers, drivers
   should use spin_lock_irqsave/spin_unlock_irqrestore to protect
   against concurrent access. This locking cannot be accomplished in
   class driver, since the lock may also be needed by the clock
   driver's interrupt service routine.

Supported hardware
==================

   * Freescale eTSEC gianfar

     - 2 Time stamp external triggers, programmable polarity (opt. interrupt)
     - 2 Alarm registers (optional interrupt)
     - 3 Periodic signals (optional interrupt)

   * National DP83640

     - 6 GPIOs programmable as inputs or outputs
     - 6 GPIOs with dedicated functions (LED/JTAG/clock) can also be
       used as general inputs or outputs
     - GPIO inputs can time stamp external triggers
     - GPIO outputs can produce periodic signals
     - 1 interrupt pin

   * Intel IXP465

     - Auxiliary Slave/Master Mode Snapshot (optional interrupt)
     - Target Time (optional interrupt)
Commit	Line	Data
c92992fc	1	.. SPDX-License-Identifier: GPL-2.0
d94ba80e	2
329f0041 MCC	3	===========================================
	4	PTP hardware clock infrastructure for Linux
	5	===========================================
d94ba80e RC	6
	7	This patch set introduces support for IEEE 1588 PTP clocks in
	8	Linux. Together with the SO_TIMESTAMPING socket options, this
	9	presents a standardized method for developing PTP user space
	10	programs, synchronizing Linux with external clocks, and using the
	11	ancillary features of PTP hardware clocks.
	12
	13	A new class driver exports a kernel interface for specific clock
	14	drivers and a user space interface. The infrastructure supports a
	15	complete set of PTP hardware clock functionality.
	16
	17	+ Basic clock operations
	18	- Set time
	19	- Get time
	20	- Shift the clock by a given offset atomically
	21	- Adjust clock frequency
	22
	23	+ Ancillary clock features
d94ba80e RC	24	- Time stamp external events
	25	- Period output signals configurable from user space
	26	- Synchronization of the Linux system time via the PPS subsystem
	27
329f0041 MCC	28	PTP hardware clock kernel API
329f0041 MCC	29	=============================
d94ba80e RC	30
	31	A PTP clock driver registers itself with the class driver. The
	32	class driver handles all of the dealings with user space. The
	33	author of a clock driver need only implement the details of
	34	programming the clock hardware. The clock driver notifies the class
	35	driver of asynchronous events (alarms and external time stamps) via
	36	a simple message passing interface.
	37
	38	The class driver supports multiple PTP clock drivers. In normal use
	39	cases, only one PTP clock is needed. However, for testing and
	40	development, it can be useful to have more than one clock in a
	41	single system, in order to allow performance comparisons.
	42
329f0041 MCC	43	PTP hardware clock user space API
329f0041 MCC	44	=================================
d94ba80e RC	45
	46	The class driver also creates a character device for each
	47	registered clock. User space can use an open file descriptor from
	48	the character device as a POSIX clock id and may call
	49	clock_gettime, clock_settime, and clock_adjtime. These calls
	50	implement the basic clock operations.
	51
	52	User space programs may control the clock using standardized
	53	ioctls. A program may query, enable, configure, and disable the
	54	ancillary clock features. User space can receive time stamped
93e9ad98	55	events via blocking read() and poll().
d94ba80e	56
329f0041 MCC	57	Writing clock drivers
329f0041 MCC	58	=====================
d94ba80e RC	59
	60	Clock drivers include include/linux/ptp_clock_kernel.h and register
	61	themselves by presenting a 'struct ptp_clock_info' to the
	62	registration method. Clock drivers must implement all of the
	63	functions in the interface. If a clock does not offer a particular
	64	ancillary feature, then the driver should just return -EOPNOTSUPP
	65	from those functions.
	66
	67	Drivers must ensure that all of the methods in interface are
	68	reentrant. Since most hardware implementations treat the time value
	69	as a 64 bit integer accessed as two 32 bit registers, drivers
	70	should use spin_lock_irqsave/spin_unlock_irqrestore to protect
	71	against concurrent access. This locking cannot be accomplished in
	72	class driver, since the lock may also be needed by the clock
	73	driver's interrupt service routine.
	74
329f0041 MCC	75	Supported hardware
	76	==================
	77
	78	* Freescale eTSEC gianfar
d94ba80e	79
d94ba80e RC	80	- 2 Time stamp external triggers, programmable polarity (opt. interrupt)
	81	- 2 Alarm registers (optional interrupt)
	82	- 3 Periodic signals (optional interrupt)
	83
329f0041 MCC	84	* National DP83640
329f0041 MCC	85
d94ba80e RC	86	- 6 GPIOs programmable as inputs or outputs
	87	- 6 GPIOs with dedicated functions (LED/JTAG/clock) can also be
	88	used as general inputs or outputs
	89	- GPIO inputs can time stamp external triggers
	90	- GPIO outputs can produce periodic signals
	91	- 1 interrupt pin
	92
329f0041 MCC	93	* Intel IXP465
329f0041 MCC	94
d94ba80e RC	95	- Auxiliary Slave/Master Mode Snapshot (optional interrupt)
d94ba80e RC	96	- Target Time (optional interrupt)