path: root/Documentation/pps
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authorRodolfo Giometti <>2009-06-17 16:28:37 -0700
committerLinus Torvalds <>2009-06-18 13:04:04 -0700
commiteae9d2ba0cfc27a2ad9765f23efb98fb80d80234 (patch)
treef4be40ca528b2f23f97fa9cb6ebe91b8d6696d5b /Documentation/pps
parent8820f27ad9a5ad2a62cdcdf425d7921c31831800 (diff)
LinuxPPS: core support
This patch adds the kernel side of the PPS support currently named "LinuxPPS". PPS means "pulse per second" and a PPS source is just a device which provides a high precision signal each second so that an application can use it to adjust system clock time. Common use is the combination of the NTPD as userland program with a GPS receiver as PPS source to obtain a wallclock-time with sub-millisecond synchronisation to UTC. To obtain this goal the userland programs shoud use the PPS API specification (RFC 2783 - Pulse-Per-Second API for UNIX-like Operating Systems, Version 1.0) which in part is implemented by this patch. It provides a set of chars devices, one per PPS source, which can be used to get the time signal. The RFC's functions can be implemented by accessing to these char devices. Signed-off-by: Rodolfo Giometti <> Cc: David Woodhouse <> Cc: Greg KH <> Cc: Randy Dunlap <> Cc: Kay Sievers <> Acked-by: Alan Cox <> Cc: Michael Kerrisk <> Cc: Christoph Hellwig <> Cc: Roman Zippel <> Signed-off-by: Andrew Morton <> Signed-off-by: Linus Torvalds <>
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diff --git a/Documentation/pps/pps.txt b/Documentation/pps/pps.txt
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+ PPS - Pulse Per Second
+ ----------------------
+(C) Copyright 2007 Rodolfo Giometti <>
+This program is free software; you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation; either version 2 of the License, or
+(at your option) any later version.
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+GNU General Public License for more details.
+LinuxPPS provides a programming interface (API) to define in the
+system several PPS sources.
+PPS means "pulse per second" and a PPS source is just a device which
+provides a high precision signal each second so that an application
+can use it to adjust system clock time.
+A PPS source can be connected to a serial port (usually to the Data
+Carrier Detect pin) or to a parallel port (ACK-pin) or to a special
+CPU's GPIOs (this is the common case in embedded systems) but in each
+case when a new pulse arrives the system must apply to it a timestamp
+and record it for userland.
+Common use is the combination of the NTPD as userland program, with a
+GPS receiver as PPS source, to obtain a wallclock-time with
+sub-millisecond synchronisation to UTC.
+RFC considerations
+While implementing a PPS API as RFC 2783 defines and using an embedded
+CPU GPIO-Pin as physical link to the signal, I encountered a deeper
+ At startup it needs a file descriptor as argument for the function
+ time_pps_create().
+This implies that the source has a /dev/... entry. This assumption is
+ok for the serial and parallel port, where you can do something
+useful besides(!) the gathering of timestamps as it is the central
+task for a PPS-API. But this assumption does not work for a single
+purpose GPIO line. In this case even basic file-related functionality
+(like read() and write()) makes no sense at all and should not be a
+precondition for the use of a PPS-API.
+The problem can be simply solved if you consider that a PPS source is
+not always connected with a GPS data source.
+So your programs should check if the GPS data source (the serial port
+for instance) is a PPS source too, and if not they should provide the
+possibility to open another device as PPS source.
+In LinuxPPS the PPS sources are simply char devices usually mapped
+into files /dev/pps0, /dev/pps1, etc..
+Coding example
+To register a PPS source into the kernel you should define a struct
+pps_source_info_s as follows:
+ static struct pps_source_info pps_ktimer_info = {
+ .name = "ktimer",
+ .path = "",
+ .echo = pps_ktimer_echo,
+ .owner = THIS_MODULE,
+ };
+and then calling the function pps_register_source() in your
+intialization routine as follows:
+ source = pps_register_source(&pps_ktimer_info,
+The pps_register_source() prototype is:
+ int pps_register_source(struct pps_source_info_s *info, int default_params)
+where "info" is a pointer to a structure that describes a particular
+PPS source, "default_params" tells the system what the initial default
+parameters for the device should be (it is obvious that these parameters
+must be a subset of ones defined in the struct
+pps_source_info_s which describe the capabilities of the driver).
+Once you have registered a new PPS source into the system you can
+signal an assert event (for example in the interrupt handler routine)
+just using:
+ pps_event(source, &ts, PPS_CAPTUREASSERT, ptr)
+where "ts" is the event's timestamp.
+The same function may also run the defined echo function
+(pps_ktimer_echo(), passing to it the "ptr" pointer) if the user
+asked for that... etc..
+Please see the file drivers/pps/clients/ktimer.c for example code.
+SYSFS support
+If the SYSFS filesystem is enabled in the kernel it provides a new class:
+ $ ls /sys/class/pps/
+ pps0/ pps1/ pps2/
+Every directory is the ID of a PPS sources defined in the system and
+inside you find several files:
+ $ ls /sys/class/pps/pps0/
+ assert clear echo mode name path subsystem@ uevent
+Inside each "assert" and "clear" file you can find the timestamp and a
+sequence number:
+ $ cat /sys/class/pps/pps0/assert
+ 1170026870.983207967#8
+Where before the "#" is the timestamp in seconds; after it is the
+sequence number. Other files are:
+* echo: reports if the PPS source has an echo function or not;
+* mode: reports available PPS functioning modes;
+* name: reports the PPS source's name;
+* path: reports the PPS source's device path, that is the device the
+ PPS source is connected to (if it exists).
+Testing the PPS support
+In order to test the PPS support even without specific hardware you can use
+the ktimer driver (see the client subsection in the PPS configuration menu)
+and the userland tools provided into Documentaion/pps/ directory.
+Once you have enabled the compilation of ktimer just modprobe it (if
+not statically compiled):
+ # modprobe ktimer
+and the run ppstest as follow:
+ $ ./ppstest /dev/pps0
+ trying PPS source "/dev/pps1"
+ found PPS source "/dev/pps1"
+ ok, found 1 source(s), now start fetching data...
+ source 0 - assert 1186592699.388832443, sequence: 364 - clear 0.000000000, sequence: 0
+ source 0 - assert 1186592700.388931295, sequence: 365 - clear 0.000000000, sequence: 0
+ source 0 - assert 1186592701.389032765, sequence: 366 - clear 0.000000000, sequence: 0
+Please, note that to compile userland programs you need the file timepps.h
+(see Documentation/pps/).