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

[Ivan Kokshaysky <ink@jurassic.park.msu.ru>]

On Fri, Jun 29, 2001 at 04:20:59PM +0400, Oleg I. Vdovikin wrote:
>     we've a bunch of UP2000/UP2000+ boards (similar to DP264) with 666MHz
> EV67 Alphas (we're building large Alpha cluster). And we're regulary see
> "HWRPB cycle frequency bogus" and the measured value for the speed in the
> range of 519 MHz - 666 MHz. And this value changes in this range from boot
> to boot. So why this happens???

This is known problem with Cypress cy82c693 SIO. The RTC on this chip
sometimes need a very long time (up to several minutes) to settle down
after reset/power-up. But I thought it's fixed on newer systems with
"ub" revision of the chip... :-(

>     So, the final question: why we're not using the aproach which is used by
> x86 time.c? I.e. why not to use CTC channel 2 for calibration?

Good idea. The patch below works reliably on my sx164.

Ivan.

--- 2.4.6-pre5/arch/alpha/kernel/time.c	Mon Nov 13 06:27:11 2000
+++ linux/arch/alpha/kernel/time.c	Fri Jun 29 20:58:09 2001
@@ -169,6 +169,63 @@ common_init_rtc(void)
 	init_rtc_irq();
 }
 
+/*
+ * Calibrate CPU clock using legacy 8254 timer/counter. Stolen from
+ * arch/i386/time.c.
+ */
+
+#define CALIBRATE_LATCH	(52 * LATCH)
+#define CALIBRATE_TIME	(52 * 1000020 / HZ)
+
+static unsigned long __init
+calibrate_cc(void)
+{
+	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 5 * LATCH count, (LSB and MSB) to begin countdown.
+	 */
+	outb(0xb0, 0x43);			/* binary, mode 0, LSB/MSB, Ch 2 */
+	outb(CALIBRATE_LATCH & 0xff, 0x42);	/* LSB of count */
+	outb(CALIBRATE_LATCH >> 8, 0x42);	/* MSB of count */
+
+	cc = rpcc();
+	do {
+		count++;
+	} while ((inb(0x61) & 0x20) == 0);
+	cc = rpcc() - cc;
+
+	/* Error: ECTCNEVERSET */
+	if (count <= 1)
+		goto bad_ctc;
+
+	/* Error: ECPUTOOFAST */
+	if (count >> 32)
+		goto bad_ctc;
+
+	/* Error: ECPUTOOSLOW */
+	if (cc <= CALIBRATE_TIME)
+		goto bad_ctc;
+
+	return ((long)cc * 1000000) / CALIBRATE_TIME;
+
+	/*
+	 * The CTC wasn't reliable: we got a hit on the very first read,
+	 * or the CPU was so fast/slow that the quotient wouldn't fit in
+	 * 32 bits..
+	 */
+bad_ctc:
+	return 0;
+}
+
 void __init
 time_init(void)
 {
@@ -176,6 +233,9 @@ time_init(void)
 	unsigned long cycle_freq, one_percent;
 	long diff;
 
+	/* Calibrate CPU clock -- attempt #1. 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