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[Xen-devel] [PATCH 4/8] x86/time: calibrate TSC against platform timer

... instead of unconditionally against the PIT. This allows for local
and master system times to remain in better sync (which matters even
when, on any modern system, the master time is really used only during
secondary CPU bringup, as the error between the two is in fact
noticable in cross-CPU NOW() invocation montonicity).

This involves moving init_platform_timer() invocation into
early_time_init(), splitting out the few things which really need to be
done in init_xen_time().

In the course of this re-ordering also set the timer channel 2 gate low
after having finished calibration. This should be benign to overall
system operation, but appears to be the more clean state.

Also do away with open coded 8254 register manipulation from 8259 code.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/i8259.c
+++ b/xen/arch/x86/i8259.c
@@ -359,12 +359,7 @@ void __init init_IRQ(void)
 
     apic_intr_init();
 
-    /* Set the clock to HZ Hz */
-#define CLOCK_TICK_RATE 1193182 /* crystal freq (Hz) */
-#define LATCH (((CLOCK_TICK_RATE)+(HZ/2))/HZ)
-    outb_p(0x34, PIT_MODE);        /* binary, mode 2, LSB/MSB, ch 0 */
-    outb_p(LATCH & 0xff, PIT_CH0); /* LSB */
-    outb(LATCH >> 8, PIT_CH0);     /* MSB */
+    preinit_pit();
 
     setup_irq(2, 0, &cascade);
 }
--- a/xen/arch/x86/time.c
+++ b/xen/arch/x86/time.c
@@ -59,7 +59,7 @@ struct platform_timesource {
     char *name;
     u64 frequency;
     u64 (*read_counter)(void);
-    int (*init)(struct platform_timesource *);
+    s64 (*init)(struct platform_timesource *);
     void (*resume)(struct platform_timesource *);
     int counter_bits;
 };
@@ -224,49 +224,18 @@ static struct irqaction __read_mostly ir
     timer_interrupt, "timer", NULL
 };
 
-/* ------ Calibrate the TSC ------- 
- * Return processor ticks per second / CALIBRATE_FRAC.
- */
-
 #define CLOCK_TICK_RATE 1193182 /* system crystal frequency (Hz) */
 #define CALIBRATE_FRAC  20      /* calibrate over 50ms */
-#define CALIBRATE_LATCH ((CLOCK_TICK_RATE+(CALIBRATE_FRAC/2))/CALIBRATE_FRAC)
+#define CALIBRATE_VALUE(freq) (((freq) + CALIBRATE_FRAC / 2) / CALIBRATE_FRAC)
 
-static u64 init_pit_and_calibrate_tsc(void)
+void preinit_pit(void)
 {
-    u64 start, end;
-    unsigned long count;
-
     /* Set PIT channel 0 to HZ Hz. */
 #define LATCH (((CLOCK_TICK_RATE)+(HZ/2))/HZ)
     outb_p(0x34, PIT_MODE);        /* binary, mode 2, LSB/MSB, ch 0 */
     outb_p(LATCH & 0xff, PIT_CH0); /* LSB */
     outb(LATCH >> 8, PIT_CH0);     /* MSB */
-
-    /* Set the Gate high, disable speaker */
-    outb((inb(0x61) & ~0x02) | 0x01, 0x61);
-
-    /*
-     * Now let's take care of CTC channel 2
-     *
-     * Set the Gate high, program CTC channel 2 for mode 0, (interrupt on
-     * terminal count mode), binary count, load 5 * LATCH count, (LSB and MSB)
-     * to begin countdown.
-     */
-    outb(0xb0, PIT_MODE);           /* binary, mode 0, LSB/MSB, Ch 2 */
-    outb(CALIBRATE_LATCH & 0xff, PIT_CH2); /* LSB of count */
-    outb(CALIBRATE_LATCH >> 8, PIT_CH2);   /* MSB of count */
-
-    start = rdtsc_ordered();
-    for ( count = 0; (inb(0x61) & 0x20) == 0; count++ )
-        continue;
-    end = rdtsc_ordered();
-
-    /* Error if the CTC doesn't behave itself. */
-    if ( count == 0 )
-        return 0;
-
-    return ((end - start) * (u64)CALIBRATE_FRAC);
+#undef LATCH
 }
 
 void set_time_scale(struct time_scale *ts, u64 ticks_per_sec)
@@ -327,10 +296,49 @@ static u64 read_pit_count(void)
     return count32;
 }
 
-static int __init init_pit(struct platform_timesource *pts)
+static s64 __init init_pit(struct platform_timesource *pts)
 {
+    u8 portb = inb(0x61);
+    u64 start, end;
+    unsigned long count;
+
     using_pit = 1;
-    return 1;
+
+    /* Set the Gate high, disable speaker. */
+    outb((portb & ~0x02) | 0x01, 0x61);
+
+    /*
+     * Now let's take care of CTC channel 2: mode 0, (interrupt on
+     * terminal count mode), binary count, load CALIBRATE_LATCH count,
+     * (LSB and MSB) to begin countdown.
+     */
+#define CALIBRATE_LATCH CALIBRATE_VALUE(CLOCK_TICK_RATE)
+    outb(0xb0, PIT_MODE);                  /* binary, mode 0, LSB/MSB, Ch 2 */
+    outb(CALIBRATE_LATCH & 0xff, PIT_CH2); /* LSB of count */
+    outb(CALIBRATE_LATCH >> 8, PIT_CH2);   /* MSB of count */
+#undef CALIBRATE_LATCH
+
+    start = rdtsc_ordered();
+    for ( count = 0; !(inb(0x61) & 0x20); ++count )
+        continue;
+    end = rdtsc_ordered();
+
+    /* Set the Gate low, disable speaker. */
+    outb(portb & ~0x03, 0x61);
+
+    /* Error if the CTC doesn't behave itself. */
+    if ( count == 0 )
+        return 0;
+
+    return (end - start) * CALIBRATE_FRAC;
+}
+
+static void resume_pit(struct platform_timesource *pts)
+{
+    /* Set CTC channel 2 to mode 0 again; initial value does not matter. */
+    outb(0xb0, PIT_MODE); /* binary, mode 0, LSB/MSB, Ch 2 */
+    outb(0, PIT_CH2);     /* LSB of count */
+    outb(0, PIT_CH2);     /* MSB of count */
 }
 
 static struct platform_timesource __initdata plt_pit =
@@ -340,7 +348,8 @@ static struct platform_timesource __init
     .frequency = CLOCK_TICK_RATE,
     .read_counter = read_pit_count,
     .counter_bits = 32,
-    .init = init_pit
+    .init = init_pit,
+    .resume = resume_pit
 };
 
 /************************************************************
@@ -352,15 +361,26 @@ static u64 read_hpet_count(void)
     return hpet_read32(HPET_COUNTER);
 }
 
-static int __init init_hpet(struct platform_timesource *pts)
+static s64 __init init_hpet(struct platform_timesource *pts)
 {
-    u64 hpet_rate = hpet_setup();
+    u64 hpet_rate = hpet_setup(), start;
+    u32 count, target;
 
     if ( hpet_rate == 0 )
         return 0;
 
     pts->frequency = hpet_rate;
-    return 1;
+
+    count = hpet_read32(HPET_COUNTER);
+    start = rdtsc_ordered();
+    target = count + CALIBRATE_VALUE(hpet_rate);
+    if ( target < count )
+        while ( hpet_read32(HPET_COUNTER) >= count )
+            continue;
+    while ( hpet_read32(HPET_COUNTER) < target )
+        continue;
+
+    return (rdtsc_ordered() - start) * CALIBRATE_FRAC;
 }
 
 static void resume_hpet(struct platform_timesource *pts)
@@ -392,12 +412,24 @@ static u64 read_pmtimer_count(void)
     return inl(pmtmr_ioport);
 }
 
-static int __init init_pmtimer(struct platform_timesource *pts)
+static s64 __init init_pmtimer(struct platform_timesource *pts)
 {
+    u64 start;
+    u32 count, target, mask = 0xffffff;
+
     if ( pmtmr_ioport == 0 )
         return 0;
 
-    return 1;
+    count = inl(pmtmr_ioport) & mask;
+    start = rdtsc_ordered();
+    target = count + CALIBRATE_VALUE(ACPI_PM_FREQUENCY);
+    if ( target < count )
+        while ( (inl(pmtmr_ioport) & mask) >= count )
+            continue;
+    while ( (inl(pmtmr_ioport) & mask) < target )
+        continue;
+
+    return (rdtsc_ordered() - start) * CALIBRATE_FRAC;
 }
 
 static struct platform_timesource __initdata plt_pmtimer =
@@ -533,14 +565,15 @@ static void resume_platform_timer(void)
     plt_stamp = plt_src.read_counter();
 }
 
-static void __init init_platform_timer(void)
+static u64 __init init_platform_timer(void)
 {
     static struct platform_timesource * __initdata plt_timers[] = {
         &plt_hpet, &plt_pmtimer, &plt_pit
     };
 
     struct platform_timesource *pts = NULL;
-    int i, rc = -1;
+    unsigned int i;
+    s64 rc = -1;
 
     if ( opt_clocksource[0] != '\0' )
     {
@@ -578,15 +611,12 @@ static void __init init_platform_timer(v
 
     plt_overflow_period = scale_delta(
         1ull << (pts->counter_bits-1), &plt_scale);
-    init_timer(&plt_overflow_timer, plt_overflow, NULL, 0);
     plt_src = *pts;
-    plt_overflow(NULL);
-
-    platform_timer_stamp = plt_stamp64;
-    stime_platform_stamp = NOW();
 
     printk("Platform timer is %s %s\n",
            freq_string(pts->frequency), pts->name);
+
+    return rc;
 }
 
 u64 stime2tsc(s_time_t stime)
@@ -1478,7 +1508,11 @@ int __init init_xen_time(void)
     /* NB. get_cmos_time() can take over one second to execute. */
     do_settime(get_cmos_time(), 0, NOW());
 
-    init_platform_timer();
+    /* Finish platform timer initialization. */
+    init_timer(&plt_overflow_timer, plt_overflow, NULL, 0);
+    plt_overflow(NULL);
+    platform_timer_stamp = plt_stamp64;
+    stime_platform_stamp = NOW();
 
     init_percpu_time();
 
@@ -1493,7 +1527,10 @@ int __init init_xen_time(void)
 void __init early_time_init(void)
 {
     struct cpu_time *t = &this_cpu(cpu_time);
-    u64 tmp = init_pit_and_calibrate_tsc();
+    u64 tmp;
+
+    preinit_pit();
+    tmp = init_platform_timer();
 
     set_time_scale(&t->tsc_scale, tmp);
     t->local_tsc_stamp = boot_tsc_stamp;
@@ -1602,7 +1639,7 @@ int time_suspend(void)
 
 int time_resume(void)
 {
-    init_pit_and_calibrate_tsc();
+    preinit_pit();
 
     resume_platform_timer();
 
--- a/xen/include/asm-x86/time.h
+++ b/xen/include/asm-x86/time.h
@@ -39,6 +39,7 @@ mktime (unsigned int year, unsigned int
 int time_suspend(void);
 int time_resume(void);
 
+void preinit_pit(void);
 void init_percpu_time(void);
 void time_latch_stamps(void);

Attachment: x86-time-TSC-calibrate-plt.patch
Description: Text document

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