pps: add parallel port PPS signal generator

Add 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.
Signed-off-by: Alexander Gordeev <lasaine@lvk.cs.msu.su>
Cc: Rodolfo Giometti <giometti@linux.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/pps/generators/Kconfig

@@ -2,16 +2,12 @@
 # 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
+	depends on PARPORT
 	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

drivers/pps/generators/pps_gen_parport.c

@@ -76,8 +76,15 @@ static enum hrtimer_restart hrtimer_event(struct hrtimer *timer)
 	long lim, delta;
 	unsigned long flags;
 
-	/* NB: approx time with blocked interrupts =
-	   send_delay + 3 * SAFETY_INTERVAL */
+	/* We have to disable interrupts here. The idea is to prevent
+	 * other interrupts on the same processor to introduce random
+	 * lags while polling the clock. getnstimeofday() takes <1us on
+	 * most machines while other interrupt handlers can take much
+	 * more potentially.
+	 *
+	 * NB: approx time with blocked interrupts =
+	 * send_delay + 3 * SAFETY_INTERVAL
+	 */
 	local_irq_save(flags);
 
 	/* first of all we get the time stamp... */