svn commit: r244290 - projects/calloutng/sys/x86/isa

[Alexander Motin]

Author: mav
Date: Sun Dec 16 14:09:18 2012
New Revision: 244290
URL: http://svnweb.freebsd.org/changeset/base/244290

Log:
  Continue fun of r244287: do not write i8254 mode register if the same value
  was already written earlier.  This reduces usleep(1) latency with i8254
  eventtimer from 22 to 19.5us.
  
  Hinted by:	bde

Modified:
  projects/calloutng/sys/x86/isa/clock.c

Modified: projects/calloutng/sys/x86/isa/clock.c
==============================================================================
--- projects/calloutng/sys/x86/isa/clock.c	Sun Dec 16 14:01:56 2012	(r244289)
+++ projects/calloutng/sys/x86/isa/clock.c	Sun Dec 16 14:09:18 2012	(r244290)
@@ -125,6 +125,7 @@ struct attimer_softc {
 static struct attimer_softc *attimer_sc = NULL;
 
 static int timer0_period = -2;
+static int timer0_mode = 0xffff;
 static int timer0_last = 0xffff;
 
 /* Values for timerX_state: */
@@ -405,7 +406,7 @@ DELAY(int n)
 static void
 set_i8254_freq(int mode, uint32_t period)
 {
-	int new_count;
+	int new_count, new_mode;
 
 	mtx_lock_spin(&clock_lock);
 	if (mode == MODE_STOP) {
@@ -424,26 +425,33 @@ set_i8254_freq(int mode, uint32_t period
 	timer0_period = (mode == MODE_PERIODIC) ? new_count : -1;
 	switch (mode) {
 	case MODE_STOP:
-		outb(TIMER_MODE, TIMER_SEL0 | TIMER_INTTC | TIMER_16BIT);
+		new_mode = TIMER_SEL0 | TIMER_INTTC | TIMER_16BIT;
+		outb(TIMER_MODE, new_mode);
 		outb(TIMER_CNTR0, 0);
 		outb(TIMER_CNTR0, 0);
 		break;
 	case MODE_PERIODIC:
-		outb(TIMER_MODE, TIMER_SEL0 | TIMER_RATEGEN | TIMER_16BIT);
+		new_mode = TIMER_SEL0 | TIMER_RATEGEN | TIMER_16BIT;
+		outb(TIMER_MODE, new_mode);
 		outb(TIMER_CNTR0, new_count & 0xff);
 		outb(TIMER_CNTR0, new_count >> 8);
 		break;
 	case MODE_ONESHOT:
 		if (new_count < 256 && timer0_last < 256) {
-			outb(TIMER_MODE, TIMER_SEL0 | TIMER_INTTC | TIMER_LSB);
+			new_mode = TIMER_SEL0 | TIMER_INTTC | TIMER_LSB;
+			if (new_mode != timer0_mode)
+				outb(TIMER_MODE, new_mode);
 			outb(TIMER_CNTR0, new_count & 0xff);
 			break;
 		}
-		outb(TIMER_MODE, TIMER_SEL0 | TIMER_INTTC | TIMER_16BIT);
+		new_mode = TIMER_SEL0 | TIMER_INTTC | TIMER_16BIT;
+		if (new_mode != timer0_mode)
+			outb(TIMER_MODE, new_mode);
 		outb(TIMER_CNTR0, new_count & 0xff);
 		outb(TIMER_CNTR0, new_count >> 8);
 		break;
 	}
+	timer0_mode = new_mode;
 	timer0_last = new_count;
 out:
 	mtx_unlock_spin(&clock_lock);
@@ -454,6 +462,8 @@ i8254_restore(void)
 {
 
 	timer0_period = -2;
+	timer0_mode = 0xffff;
+	timer0_last = 0xffff;
 	if (attimer_sc != NULL)
 		set_i8254_freq(attimer_sc->mode, attimer_sc->period);
 	else