1 files changed, 65 insertions, 45 deletions

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 83d53ce5d4c4..462b9ba67e92 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -273,30 +273,43 @@ static unsigned long pit_calibrate_tsc(u32 latch, unsigned long ms, int loopmin)
  * use the TSC value at the transitions to calculate a pretty
  * good value for the TSC frequencty.
  */
-static inline int pit_expect_msb(unsigned char val)
+static inline int pit_expect_msb(unsigned char val, u64 *tscp, unsigned long *deltap)
 {
-        int count = 0;
+        int count;
+        u64 tsc = 0;
 
         for (count = 0; count < 50000; count++) {
                 /* Ignore LSB */
                 inb(0x42);
                 if (inb(0x42) != val)
                         break;
+                tsc = get_cycles();
         }
-        return count > 50;
+        *deltap = get_cycles() - tsc;
+        *tscp = tsc;
+
+        /*
+         * We require _some_ success, but the quality control
+         * will be based on the error terms on the TSC values.
+         */
+        return count > 5;
 }
 
 /*
- * How many MSB values do we want to see? We aim for a
- * 15ms calibration, which assuming a 2us counter read
- * error should give us roughly 150 ppm precision for
- * the calibration.
+ * How many MSB values do we want to see? We aim for
+ * a maximum error rate of 500ppm (in practice the
+ * real error is much smaller), but refuse to spend
+ * more than 25ms on it.
  */
-#define QUICK_PIT_MS 15
-#define QUICK_PIT_ITERATIONS (QUICK_PIT_MS * PIT_TICK_RATE / 1000 / 256)
+#define MAX_QUICK_PIT_MS 25
+#define MAX_QUICK_PIT_ITERATIONS (MAX_QUICK_PIT_MS * PIT_TICK_RATE / 1000 / 256)
 
 static unsigned long quick_pit_calibrate(void)
 {
+        int i;
+        u64 tsc, delta;
+        unsigned long d1, d2;
+
         /* Set the Gate high, disable speaker */
         outb((inb(0x61) & ~0x02) | 0x01, 0x61);
 
@@ -315,45 +328,52 @@ static unsigned long quick_pit_calibrate(void)
         outb(0xff, 0x42);
         outb(0xff, 0x42);
 
-        if (pit_expect_msb(0xff)) {
-                int i;
-                u64 t1, t2, delta;
-                unsigned char expect = 0xfe;
-
-                t1 = get_cycles();
-                for (i = 0; i < QUICK_PIT_ITERATIONS; i++, expect--) {
-                        if (!pit_expect_msb(expect))
-                                goto failed;
+        /*
+         * The PIT starts counting at the next edge, so we
+         * need to delay for a microsecond. The easiest way
+         * to do that is to just read back the 16-bit counter
+         * once from the PIT.
+         */
+        inb(0x42);
+        inb(0x42);
+
+        if (pit_expect_msb(0xff, &tsc, &d1)) {
+                for (i = 1; i <= MAX_QUICK_PIT_ITERATIONS; i++) {
+                        if (!pit_expect_msb(0xff-i, &delta, &d2))
+                                break;
+
+                        /*
+                         * Iterate until the error is less than 500 ppm
+                         */
+                        delta -= tsc;
+                        if (d1+d2 < delta >> 11)
+                                goto success;
                 }
-                t2 = get_cycles();
-
-                /*
-                 * Make sure we can rely on the second TSC timestamp:
-                 */
-                if (!pit_expect_msb(expect))
-                        goto failed;
-
-                /*
-                 * Ok, if we get here, then we've seen the
-                 * MSB of the PIT decrement QUICK_PIT_ITERATIONS
-                 * times, and each MSB had many hits, so we never
-                 * had any sudden jumps.
-                 *
-                 * As a result, we can depend on there not being
-                 * any odd delays anywhere, and the TSC reads are
-                 * reliable.
-                 *
-                 * kHz = ticks / time-in-seconds / 1000;
-                 * kHz = (t2 - t1) / (QPI * 256 / PIT_TICK_RATE) / 1000
-                 * kHz = ((t2 - t1) * PIT_TICK_RATE) / (QPI * 256 * 1000)
-                 */
-                delta = (t2 - t1)*PIT_TICK_RATE;
-                do_div(delta, QUICK_PIT_ITERATIONS*256*1000);
-                printk("Fast TSC calibration using PIT\n");
-                return delta;
         }
-failed:
+        printk("Fast TSC calibration failed\n");
         return 0;
+
+success:
+        /*
+         * Ok, if we get here, then we've seen the
+         * MSB of the PIT decrement 'i' times, and the
+         * error has shrunk to less than 500 ppm.
+         *
+         * As a result, we can depend on there not being
+         * any odd delays anywhere, and the TSC reads are
+         * reliable (within the error). We also adjust the
+         * delta to the middle of the error bars, just
+         * because it looks nicer.
+         *
+         * kHz = ticks / time-in-seconds / 1000;
+         * kHz = (t2 - t1) / (I * 256 / PIT_TICK_RATE) / 1000
+         * kHz = ((t2 - t1) * PIT_TICK_RATE) / (I * 256 * 1000)
+         */
+        delta += (long)(d2 - d1)/2;
+        delta *= PIT_TICK_RATE;
+        do_div(delta, i*256*1000);
+        printk("Fast TSC calibration using PIT\n");
+        return delta;
 }
 
 /**