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SubjectRe: alpha - generic_init_pit - why using RTC for calibration?
Date
    Here is the patch against the buggy Cypress RTC which is found on some
Alpha boards. It's tested with 2.2.16 & 2.2.19 kernels and as seems should
work with 2.4.x kernels. This patch differs from initial Ivan's version by
the "cc" variable type & different calibrate divisor usage for better
accuracy.

Please consider applying this patch against the 2.2.x tree. It's really
needed due to overall performance reasons.

Thanks,
Oleg.

--- linux/arch/alpha/kernel/time.c.orig Mon Jul 2 14:05:09 2001
+++ linux/arch/alpha/kernel/time.c Mon Jul 2 15:47:45 2001
@@ -231,6 +231,49 @@
outb(0x13, 0x42);
}

+/*
+ * Calibrate CPU clock using legacy 8254 timer/counter. Stolen from
+ * arch/i386/time.c.
+ */
+
+#define CALIBRATE_DIVISOR 0xffff
+
+static unsigned long __init
+calibrate_cc(void)
+{
+ unsigned int cc;
+ unsigned long count = 0;
+
+ /* 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 maximum divisor we can get for accuracy - 65535
+ */
+
+ outb(0xb0, 0x43); /* binary, mode 0, LSB/MSB, Ch 2 */
+ outb(CALIBRATE_DIVISOR & 0xff, 0x42); /* LSB of count */
+ outb(CALIBRATE_DIVISOR >> 8, 0x42); /* MSB of count */
+
+ /* we still should not hang if timer not runing or N/A */
+ for (cc = rpcc(); (inb(0x61) & 0x20) == 0 && !(count >> 32); count++);
+
+ /* cycles delta */
+ cc = rpcc() - cc;
+
+ /* check for the reliable result */
+ if ((count < 1) || (count >> 32))
+ return 0;
+
+ /* and the final result in HZ */
+ return ((unsigned long)cc * CLOCK_TICK_RATE) / CALIBRATE_DIVISOR;
+}
+
+
void
time_init(void)
{
@@ -239,6 +282,10 @@
unsigned long cycle_freq, ppm_error;
long diff;

+ /* Calibrate CPU clock using CTC. If this fails, use RTC. */
+ if (!est_cycle_freq)
+ est_cycle_freq = calibrate_cc();
+
/*
* The Linux interpretation of the CMOS clock register contents:
* When the Update-In-Progress (UIP) flag goes from 1 to 0, the

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