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