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

Ivan Kokshaysky
2001-06-29T17:20:30Z

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