docs: pps.txt: convert to ReST and rename to pps.rst

This file is already in a good shape: just its title and
adding some literal block markups is needed for it to be
part of the document.

While it has a small chapter with sysfs stuff, most of
the document is focused on driver development.

As it describes a kernel API, move it to the driver-api
directory.

In order to avoid conflicts, let's add an :orphan: tag
to it, to be removed when added to the driver-api book.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Rodolfo Giometti <giometti@enneenne.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

1 files changed, 242 insertions, 0 deletions

diff --git a/Documentation/driver-api/pps.rst b/Documentation/driver-api/pps.rst
new file mode 100644
index 000000000000..1456d2c32ebd
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+:orphan:
+
+======================
+PPS - Pulse Per Second
+======================
+
+Copyright (C) 2007 Rodolfo Giometti <giometti@enneenne.com>
+
+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
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+
+
+Overview
+--------
+
+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
+problem:
+
+   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.
+
+
+PPS with USB to serial devices
+------------------------------
+
+It is possible to grab the PPS from an USB to serial device. However,
+you should take into account the latencies and jitter introduced by
+the USB stack. Users have reported clock instability around +-1ms when
+synchronized with PPS through USB. With USB 2.0, jitter may decrease
+down to the order of 125 microseconds.
+
+This may be suitable for time server synchronization with NTP because
+of its undersampling and algorithms.
+
+If your device doesn't report PPS, you can check that the feature is
+supported by its driver. Most of the time, you only need to add a call
+to usb_serial_handle_dcd_change after checking the DCD status (see
+ch341 and pl2303 examples).
+
+
+Coding example
+--------------
+
+To register a PPS source into the kernel you should define a struct
+pps_source_info as follows::
+
+    static struct pps_source_info pps_ktimer_info = {
+	    .name         = "ktimer",
+	    .path         = "",
+	    .mode         = PPS_CAPTUREASSERT | PPS_OFFSETASSERT |
+			    PPS_ECHOASSERT |
+			    PPS_CANWAIT | PPS_TSFMT_TSPEC,
+	    .echo         = pps_ktimer_echo,
+	    .owner        = THIS_MODULE,
+    };
+
+and then calling the function pps_register_source() in your
+initialization routine as follows::
+
+    source = pps_register_source(&pps_ktimer_info,
+			PPS_CAPTUREASSERT | PPS_OFFSETASSERT);
+
+The pps_register_source() prototype is::
+
+  int pps_register_source(struct pps_source_info *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 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/pps-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 -F /sys/class/pps/pps0/
+   assert     dev        mode       path       subsystem@
+   clear      echo       name       power/     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 pps-ktimer driver (see the client subsection in the PPS configuration menu)
+and the userland tools available in your distribution's pps-tools package,
+http://linuxpps.org , or https://github.com/redlab-i/pps-tools.
+
+Once you have enabled the compilation of pps-ktimer just modprobe it (if
+not statically compiled)::
+
+   # modprobe pps-ktimer
+
+and the run ppstest as follow::
+
+   $ ./ppstest /dev/pps1
+   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.
+This is available in the pps-tools repository mentioned above.
+
+
+Generators
+----------
+
+Sometimes one needs to be able not only to catch PPS signals but to produce
+them also. For example, running a distributed simulation, which requires
+computers' clock to be synchronized very tightly. One way to do this is to
+invent some complicated hardware solutions but it may be neither necessary
+nor affordable. The cheap way is to load a PPS generator on one of the
+computers (master) and PPS clients on others (slaves), and use very simple
+cables to deliver signals using parallel ports, for example.
+
+Parallel port cable pinout::
+
+	pin	name	master      slave
+	1	STROBE	  *------     *
+	2	D0	  *     |     *
+	3	D1	  *     |     *
+	4	D2	  *     |     *
+	5	D3	  *     |     *
+	6	D4	  *     |     *
+	7	D5	  *     |     *
+	8	D6	  *     |     *
+	9	D7	  *     |     *
+	10	ACK	  *     ------*
+	11	BUSY	  *           *
+	12	PE	  *           *
+	13	SEL	  *           *
+	14	AUTOFD	  *           *
+	15	ERROR	  *           *
+	16	INIT	  *           *
+	17	SELIN	  *           *
+	18-25	GND	  *-----------*
+
+Please note that parallel port interrupt occurs only on high->low transition,
+so it is used for PPS assert edge. PPS clear edge can be determined only
+using polling in the interrupt handler which actually can be done way more
+precisely because interrupt handling delays can be quite big and random. So
+current parport PPS generator implementation (pps_gen_parport module) is
+geared towards using the clear edge for time synchronization.
+
+Clear edge polling is done with disabled interrupts so it's better to select
+delay between assert and clear edge as small as possible to reduce system
+latencies. But if it is too small slave won't be able to capture clear edge
+transition. The default of 30us should be good enough in most situations.
+The delay can be selected using 'delay' pps_gen_parport module parameter.