pps: add parallel port PPS signal generator

Please, note that to compile userland programs you need the file timepps.h

(see Documentation/pps/).

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.

source drivers/pps/clients/Kconfig

source drivers/pps/generators/Kconfig

endmenu

pps_core-y			:= pps.o kapi.o sysfs.o

pps_core-$(CONFIG_NTP_PPS)	+= kc.o

obj-$(CONFIG_PPS)		:= pps_core.o

obj-y				+= clients/

obj-y				+= clients/ generators/

ccflags-$(CONFIG_PPS_DEBUG) := -DDEBUG

#

# PPS generators configuration

#

if PPS

comment "PPS generators support"

config PPS_GENERATOR_PARPORT

	tristate "Parallel port PPS signal generator"

	depends on PARPORT != n && GENERIC_TIME

	help

	  If you say yes here you get support for a PPS signal generator which

	  utilizes STROBE pin of a parallel port to send PPS signals. It uses

	  parport abstraction layer and hrtimers to precisely control the signal.

endif

#

# Makefile for PPS generators.

#

obj-$(CONFIG_PPS_GENERATOR_PARPORT) += pps_gen_parport.o

ifeq ($(CONFIG_PPS_DEBUG),y)

EXTRA_CFLAGS += -DDEBUG

endif