From c763b22a79ef10c0a961b24ae617980231681e06 Mon Sep 17 00:00:00 2001 From: phk Date: Wed, 26 Mar 2008 22:12:00 +0000 Subject: Back in the good old days, PC's had random pieces of rock for frequency generation and what frequency the generated was anyones guess. In general the 32.768kHz RTC clock x-tal was the best, because that was a regular wrist-watch Xtal, whereas the X-tal generating the ISA bus frequency was much lower quality, often costing as much as several cents a piece, so it made good sense to check the ISA bus frequency against the RTC clock. The other relevant property of those machines, is that they typically had no more than 16MB RAM. These days, CPU chips croak if their clocks are not tightly within specs and all necessary frequencies are derived from the master crystal by means if PLL's. Considering that it takes on average 1.5 second to calibrate the frequency of the i8254 counter, that more likely than not, we will not actually use the result of the calibration, and as the final clincher, we seldom use the i8254 for anything besides BEL in syscons anyway, it has become time to drop the calibration code. If you need to tell the system what frequency your i8254 runs, you can do so from the loader using hw.i8254.freq or using the sysctl kern.timecounter.tc.i8254.frequency. --- sys/amd64/conf/NOTES | 13 ------------- 1 file changed, 13 deletions(-) (limited to 'sys/amd64/conf') diff --git a/sys/amd64/conf/NOTES b/sys/amd64/conf/NOTES index bfa1c9b..9be763e 100644 --- a/sys/amd64/conf/NOTES +++ b/sys/amd64/conf/NOTES @@ -88,19 +88,6 @@ options BPF_JITTER ##################################################################### # CLOCK OPTIONS -# The following options are used for debugging clock behavior only, and -# should not be used for production systems. - -# CLK_CALIBRATION_LOOP causes clock calibration to be run in a loop at -# startup until the user presses a key. (The i8254 clock is always -# calibrated relative to the RTC (mc146818a) and this option causes the -# calibration to be repeated.) -options CLK_CALIBRATION_LOOP - -# CLK_USE_I8254_CALIBRATION causes the calibrated frequency of the i8254 -# clock to actually be used. -options CLK_USE_I8254_CALIBRATION - # Provide read/write access to the memory in the clock chip. device nvram # Access to rtc cmos via /dev/nvram -- cgit v1.1