MIPS: DECstation HRT calibration bug fixes

Maciej W. Rozycki · ralfbaechle · commit 8533966a6dba · 2013-09-05T20:38:28.000+02:00
This change corrects DECstation HRT calibration, by removing the following bugs: 1. Calibration period selection -- HZ / 10 has been chosen, however on DECstation computers, HZ never divides by 10, as the choice for HZ is among 128, 256 and 1024. The choice therefore results in a systematic calibration error, e.g. 6.25% for the usual choice of 128 for HZ: 128 / 10 * 10 = 120 (128 - 120) / 128 -> 6.25% The change therefore makes calibration use HZ / 8 that is always accurate for the HZ values available, getting rid of the systematic error. 2. Calibration starting point synchronisation -- the duration of a number of intervals between DS1287A periodic interrupt assertions is measured, however code does not ensure at the beginning that the interrupt has not been previously asserted. This results in a variable error of e.g. up to another 6.25% for the period of HZ / 8 (8.(3)% with the original HZ / 10 period) and the usual choice of 128 for HZ: 1 / 16 -> 6.25% 1 / 12 -> 8.(3)% The change therefore adds an initial call to ds1287_timer_state that clears any previous periodic interrupt pending. The same issue applies to both I/O ASIC counter and R4k CP0 timer calibration on DECstation systems as similar code is used in both cases and both pieces of code are covered by this fix. On an R3400 test system used this fix results in a change of the I/O ASIC clock frequency reported from values like: I/O ASIC clock frequency 23185830Hz to: I/O ASIC clock frequency 24999288Hz removing the miscalculation by 6.25% from the systematic error and (for the individual sample provided) a further 1.00% from the variable error, accordingly. The nominal I/O ASIC clock frequency is 25MHz on this system. Here's another result, with the fix applied, from a system that has both HRTs available (using an R4400 at 60MHz nominal): MIPS counter frequency 59999328Hz I/O ASIC clock frequency 24999432Hz Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org> Cc: linux-mips@linux-mips.org Patchwork: https://patchwork.linux-mips.org/patch/5807/ Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
diff --git a/arch/mips/dec/time.c b/arch/mips/dec/time.c
@@ -126,12 +126,13 @@ int rtc_mips_set_mmss(unsigned long nowtime)
 void __init plat_time_init(void)
 {
 	u32 start, end;
-	int i = HZ / 10;
+	int i = HZ / 8;
 
 	/* Set up the rate of periodic DS1287 interrupts. */
 	ds1287_set_base_clock(HZ);
 
 	if (cpu_has_counter) {
+		ds1287_timer_state();
 		while (!ds1287_timer_state())
 			;
 
@@ -143,7 +144,7 @@ void __init plat_time_init(void)
 
 		end = read_c0_count();
 
-		mips_hpt_frequency = (end - start) * 10;
+		mips_hpt_frequency = (end - start) * 8;
 		printk(KERN_INFO "MIPS counter frequency %dHz\n",
 			mips_hpt_frequency);
 	} else if (IOASIC)
diff --git a/arch/mips/kernel/csrc-ioasic.c b/arch/mips/kernel/csrc-ioasic.c
@@ -41,9 +41,9 @@ void __init dec_ioasic_clocksource_init(void)
 {
 	unsigned int freq;
 	u32 start, end;
-	int i = HZ / 10;
-
+	int i = HZ / 8;
 
+	ds1287_timer_state();
 	while (!ds1287_timer_state())
 		;
 
@@ -55,7 +55,7 @@ void __init dec_ioasic_clocksource_init(void)
 
 	end = dec_ioasic_hpt_read(&clocksource_dec);
 
-	freq = (end - start) * 10;
+	freq = (end - start) * 8;
 	printk(KERN_INFO "I/O ASIC clock frequency %dHz\n", freq);
 
 	clocksource_dec.rating = 200 + freq / 10000000;
