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c92992fc60
There are a number of driver-specific descriptions that contain a mix of userspace and kernelspace documentation. Just like we did with other similar subsystems, add them at the driver-api groupset, but don't move the directories. Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
97 lines
3.7 KiB
ReStructuredText
97 lines
3.7 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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===========================================
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PTP hardware clock infrastructure for Linux
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===========================================
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This patch set introduces support for IEEE 1588 PTP clocks in
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Linux. Together with the SO_TIMESTAMPING socket options, this
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presents a standardized method for developing PTP user space
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programs, synchronizing Linux with external clocks, and using the
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ancillary features of PTP hardware clocks.
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A new class driver exports a kernel interface for specific clock
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drivers and a user space interface. The infrastructure supports a
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complete set of PTP hardware clock functionality.
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+ Basic clock operations
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- Set time
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- Get time
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- Shift the clock by a given offset atomically
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- Adjust clock frequency
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+ Ancillary clock features
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- Time stamp external events
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- Period output signals configurable from user space
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- Synchronization of the Linux system time via the PPS subsystem
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PTP hardware clock kernel API
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=============================
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A PTP clock driver registers itself with the class driver. The
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class driver handles all of the dealings with user space. The
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author of a clock driver need only implement the details of
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programming the clock hardware. The clock driver notifies the class
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driver of asynchronous events (alarms and external time stamps) via
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a simple message passing interface.
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The class driver supports multiple PTP clock drivers. In normal use
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cases, only one PTP clock is needed. However, for testing and
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development, it can be useful to have more than one clock in a
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single system, in order to allow performance comparisons.
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PTP hardware clock user space API
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=================================
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The class driver also creates a character device for each
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registered clock. User space can use an open file descriptor from
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the character device as a POSIX clock id and may call
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clock_gettime, clock_settime, and clock_adjtime. These calls
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implement the basic clock operations.
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User space programs may control the clock using standardized
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ioctls. A program may query, enable, configure, and disable the
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ancillary clock features. User space can receive time stamped
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events via blocking read() and poll().
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Writing clock drivers
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=====================
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Clock drivers include include/linux/ptp_clock_kernel.h and register
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themselves by presenting a 'struct ptp_clock_info' to the
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registration method. Clock drivers must implement all of the
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functions in the interface. If a clock does not offer a particular
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ancillary feature, then the driver should just return -EOPNOTSUPP
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from those functions.
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Drivers must ensure that all of the methods in interface are
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reentrant. Since most hardware implementations treat the time value
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as a 64 bit integer accessed as two 32 bit registers, drivers
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should use spin_lock_irqsave/spin_unlock_irqrestore to protect
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against concurrent access. This locking cannot be accomplished in
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class driver, since the lock may also be needed by the clock
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driver's interrupt service routine.
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Supported hardware
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==================
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* Freescale eTSEC gianfar
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- 2 Time stamp external triggers, programmable polarity (opt. interrupt)
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- 2 Alarm registers (optional interrupt)
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- 3 Periodic signals (optional interrupt)
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* National DP83640
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- 6 GPIOs programmable as inputs or outputs
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- 6 GPIOs with dedicated functions (LED/JTAG/clock) can also be
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used as general inputs or outputs
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- GPIO inputs can time stamp external triggers
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- GPIO outputs can produce periodic signals
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- 1 interrupt pin
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* Intel IXP465
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- Auxiliary Slave/Master Mode Snapshot (optional interrupt)
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- Target Time (optional interrupt)
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