Re: alpha - generic_init_pit - why using RTC for calibration?

["Oleg I. Vdovikin" <vdovikin@jscc.ru>]

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    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



[-- Attachment #2: linux-2.2.x.rtc.patch --]
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--- 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
@@ -275,7 +322,7 @@
 	if (diff < 0)
 		diff = -diff;
 	ppm_error = (diff * 1000000L) / cycle_freq;
-#if 0
+#if 1
 	printk("Alpha clock init: HWRPB %lu, Measured %lu, error=%lu ppm.\n",
 	       hwrpb->cycle_freq, est_cycle_freq, ppm_error);
 #endif