linux-kernel - [PATCH 0/5] pps: time synchronization over LPT

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Message-Id: <cover.1265228858.git.lasaine@lvk.cs.msu.su>
Date:	Wed,  3 Feb 2010 23:56:46 +0300
From:	Alexander Gordeev <lasaine@....cs.msu.su>
To:	linux-kernel@...r.kernel.org
Cc:	linuxpps@...enneenne.com,
	"Nikita V\. Youshchenko" <yoush@...msu.su>, stas@....cs.msu.su,
	Alexander Gordeev <lasaine@....cs.msu.su>
Subject: [PATCH 0/5] pps: time synchronization over LPT

This patchset adds a complete solution (well, the kernel part) for fast
high-precision time synchronization between computers using their LPT
ports. My tests show that it is able to achieve synchronization in the
order of 1-2 microseconds and keep it for as long as needed. We are
using this facility in my company for distributed real-time simulation.

The idea is to have a single master computer whose local clock is used
as a standard. It then provides time information through both PPS
high-precision signal over LPT and NTP over network. Then any other
computer can combine these two time sources (this is what ntpd can do).
Client-side ntpd constantly checks that the local time divergence from
the master's time is less then 128ms. If this is true it uses PPS
signal to further decrease the difference. ntpd can try to do itself
but it's too slow and imprecise. This is really a job for the kernel.

All stuff related to the PPS signals in Linux kernel is handled by the
code from LinuxPPS project. It can be controlled from user-space
through PPSAPI (RFC 2783) and ntpd can use it to pass control over
synchronization to the kernel. Unfortunately this is an optional
feature of PPSAPI and it was not implemented. Also Linux code from
NTPv4 reference model was missing all the pieces that deal with PPS.
This patchset closes both gaps to achieve the highest precision and
speed. It:

 * adds missing hardpps() function to the NTPv4 Linux kernel code

 * adds getnstime_raw_and_real() to atomically get both MONOTONIC_RAW
   and REALTIME clock timestamps for hardpps()

 * adds missing optional PPSAPI method to bind the particular PPS
   source to feed information to the time subsystem through hardpps()

Also the actual parallel port PPS generator and client were added. (I
know that not yet mainlined LinuxPPS patches provide an own parallel
port client but it is actually a hack on lp driver which I didn't like
at all.)

This patchset is tested against the vanilla 2.6.32.7 kernel. But we are
actually using it on 2.6.31.12-rt20 rt-preempt kernel most of the time.
Also there is a version which should be applied on top of LinuxPPS out
of tree patches (i.e. all clients and low-level irq timestamps stuff).
Those who are interested in other versions of the patchset can find
them in my git repository:
http://lvk.cs.msu.su/~lasaine/timesync/linux-2.6-timesync.git

Alexander Gordeev (5):
  ntp: add hardpps implementation
  pps: capture MONOTONIC_RAW timestamps as well
  pps: add kernel consumer support
  pps: add parallel port PPS signal generator
  pps: add parallel port PPS client

 Documentation/ioctl/ioctl-number.txt     |    2 +-
 drivers/pps/Kconfig                      |    5 +
 drivers/pps/Makefile                     |    1 +
 drivers/pps/clients/Kconfig              |   16 ++
 drivers/pps/clients/Makefile             |    9 +
 drivers/pps/clients/pps_parport.c        |  211 ++++++++++++++++++
 drivers/pps/generators/Kconfig           |   17 ++
 drivers/pps/generators/Makefile          |    9 +
 drivers/pps/generators/pps_gen_parport.c |  263 +++++++++++++++++++++++
 drivers/pps/kapi.c                       |   45 ++++-
 drivers/pps/pps.c                        |   67 ++++++-
 include/linux/pps.h                      |    7 +
 include/linux/pps_kernel.h               |   39 ++++-
 include/linux/time.h                     |    2 +
 include/linux/timex.h                    |    1 +
 kernel/time/Kconfig                      |    7 +
 kernel/time/ntp.c                        |  346 +++++++++++++++++++++++++++++-
 kernel/time/timekeeping.c                |   34 +++
 18 files changed, 1065 insertions(+), 16 deletions(-)
 create mode 100644 drivers/pps/clients/Kconfig
 create mode 100644 drivers/pps/clients/Makefile
 create mode 100644 drivers/pps/clients/pps_parport.c
 create mode 100644 drivers/pps/generators/Kconfig
 create mode 100644 drivers/pps/generators/Makefile
 create mode 100644 drivers/pps/generators/pps_gen_parport.c

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