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)
