[PATCH gnumach] lapic timer: Calibrate based on 10 PIT sleeps for 10ms

[Damien Zammit]

NB: This relies on a fix for QEMU as one-shot PIT mode
is currently broken in qemu.
---
 i386/i386at/ioapic.c | 19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

diff --git a/i386/i386at/ioapic.c b/i386/i386at/ioapic.c
index 24e04653..d2ea84ad 100644
--- a/i386/i386at/ioapic.c
+++ b/i386/i386at/ioapic.c
@@ -151,8 +151,9 @@ ioapic_toggle_entry(int apic, int pin, int mask)
 }

 static uint32_t
-pit_measure_apic_hz(void)
+pit_measure_10x_apic_hz(void)
 {
+    volatile int i;
     uint32_t start = 0xffffffff;

     /* Prepare accurate delay for 1/hz seconds */
@@ -162,7 +163,8 @@ pit_measure_apic_hz(void)
     lapic->init_count.r = start;

     /* zZz */
-    pit_sleep();
+    for (i = 0; i < 10; i++)
+        pit_sleep();

     /* Stop APIC timer */
     lapic->lvt_timer.r |= LAPIC_DISABLE;
@@ -181,13 +183,13 @@ lapic_enable_timer(void)
 {
     /* Set up counter */
     lapic->init_count.r = calibrated_ticks;
-    lapic->divider_config.r = LAPIC_TIMER_DIVIDE_16;
+    lapic->divider_config.r = LAPIC_TIMER_DIVIDE_2;

     /* Set the timer to interrupt periodically on remapped timer GSI */
     lapic->lvt_timer.r = IOAPIC_INT_BASE | LAPIC_TIMER_PERIODIC;

     /* Some buggy hardware requires this set again */
-    lapic->divider_config.r = LAPIC_TIMER_DIVIDE_16;
+    lapic->divider_config.r = LAPIC_TIMER_DIVIDE_2;

     /* Enable interrupts for the first time */
     printf("LAPIC timer configured on cpu%d\n", cpu_number());
@@ -336,12 +338,13 @@ ioapic_configure(void)
     lapic_enable();

     /* Set one-shot timer */
-    lapic->divider_config.r = LAPIC_TIMER_DIVIDE_16;
+    lapic->divider_config.r = LAPIC_TIMER_DIVIDE_2;
     lapic->lvt_timer.r = IOAPIC_INT_BASE;

-    /* Measure number of APIC timer ticks in 1/hz seconds
-     * but calibrate the timer to expire at rate of hz */
-    calibrated_ticks = pit_measure_apic_hz() * hz;
+    /* Measure number of APIC timer ticks in 10x 1/hz seconds
+     * but calibrate the timer to expire at rate of hz
+     * divide by 10 because we waited 10 times longer than we needed */
+    calibrated_ticks = pit_measure_10x_apic_hz() / 10;

     /* Set up counter later */
     lapic->lvt_timer.r = LAPIC_DISABLE;
--
2.39.0