[MIPS] Handle R4000/R4400 mfc0 from count register.

The R4000 and R4400 have an errata where if the cp0 count register is read
in the exact moment when it matches the compare register no interrupt will
be generated.

This bug may be triggered if the cp0 count register is being used as
clocksource and the compare interrupt as clockevent.  So a simple
workaround is to avoid using the compare for both facilities on the
affected CPUs.

This is different from the workaround suggested in the old errata documents;
at some opportunity probably the official version should be implemented
and tested.  Another thing to find out is which processor versions
exactly are affected.  I only have errata documents upto R4400 V3.0
available so for the moment the code treats all R4000 and R4400 as broken.

This is potencially a problem for some machines that have no other decent
clocksource available; this workaround will cause them to fall back to
another clocksource, worst case the "jiffies" source.

arch/mips/kernel/cevt-r4k.c

@@ -219,7 +219,7 @@ static int c0_compare_int_usable(void)
 	return 1;
 }
 
-void __cpuinit mips_clockevent_init(void)
+int __cpuinit mips_clockevent_init(void)
 {
 	uint64_t mips_freq = mips_hpt_frequency;
 	unsigned int cpu = smp_processor_id();
@@ -227,7 +227,7 @@ void __cpuinit mips_clockevent_init(void)
 	unsigned int irq;
 
 	if (!cpu_has_counter || !mips_hpt_frequency)
-		return;
+		return -ENXIO;
 
 #ifdef CONFIG_MIPS_MT_SMTC
 	setup_smtc_dummy_clockevent_device();
@@ -237,11 +237,11 @@ void __cpuinit mips_clockevent_init(void)
 	 * device.
 	 */
 	if (cpu)
-		return;
+		return 0;
 #endif
 
 	if (!c0_compare_int_usable())
-		return;
+		return -ENXIO;
 
 	/*
 	 * With vectored interrupts things are getting platform specific.
@@ -277,7 +277,7 @@ void __cpuinit mips_clockevent_init(void)
 	clockevents_register_device(cd);
 
 	if (cp0_timer_irq_installed)
-		return;
+		return 0;
 
 	cp0_timer_irq_installed = 1;
 
@@ -287,4 +287,6 @@ void __cpuinit mips_clockevent_init(void)
 #else
 	setup_irq(irq, &c0_compare_irqaction);
 #endif
+
+	return 0;
 }

arch/mips/kernel/time.c

@@ -91,48 +91,6 @@ static struct clocksource clocksource_mips = {
 	.flags		= CLOCK_SOURCE_IS_CONTINUOUS,
 };
 
-static unsigned int __init calibrate_hpt(void)
-{
-	cycle_t frequency, hpt_start, hpt_end, hpt_count, hz;
-
-	const int loops = HZ / 10;
-	int log_2_loops = 0;
-	int i;
-
-	/*
-	 * We want to calibrate for 0.1s, but to avoid a 64-bit
-	 * division we round the number of loops up to the nearest
-	 * power of 2.
-	 */
-	while (loops > 1 << log_2_loops)
-		log_2_loops++;
-	i = 1 << log_2_loops;
-
-	/*
-	 * Wait for a rising edge of the timer interrupt.
-	 */
-	while (mips_timer_state());
-	while (!mips_timer_state());
-
-	/*
-	 * Now see how many high precision timer ticks happen
-	 * during the calculated number of periods between timer
-	 * interrupts.
-	 */
-	hpt_start = clocksource_mips.read();
-	do {
-		while (mips_timer_state());
-		while (!mips_timer_state());
-	} while (--i);
-	hpt_end = clocksource_mips.read();
-
-	hpt_count = (hpt_end - hpt_start) & clocksource_mips.mask;
-	hz = HZ;
-	frequency = hpt_count * hz;
-
-	return frequency >> log_2_loops;
-}
-
 void __init clocksource_set_clock(struct clocksource *cs, unsigned int clock)
 {
 	u64 temp;
@@ -194,21 +152,42 @@ void __init plat_timer_setup(void)
 	BUG();
 }
 
-void __init time_init(void)
+static __init int cpu_has_mfc0_count_bug(void)
 {
-	plat_time_init();
+	switch (current_cpu_type()) {
+	case CPU_R4000PC:
+	case CPU_R4000SC:
+	case CPU_R4000MC:
+		/*
+		 * V3.0 is documented as suffering from the mfc0 from count bug.
+		 * Afaik this is the last version of the R4000.  Later versions
+		 * were marketed as R4400.
+		 */
+		return 1;
 
-	if (cpu_has_counter && (mips_hpt_frequency || mips_timer_state)) {
-		/* We know counter frequency.  Or we can get it.  */
-		if (!mips_hpt_frequency)
-			mips_hpt_frequency = calibrate_hpt();
+	case CPU_R4400PC:
+	case CPU_R4400SC:
+	case CPU_R4400MC:
+		/*
+		 * The published errata for the R4400 upto 3.0 say the CPU
+		 * has the mfc0 from count bug.
+		 */
+		if ((current_cpu_data.processor_id & 0xff) <= 0x30)
+			return 1;
 
-		/* Report the high precision timer rate for a reference.  */
-		printk("Using %u.%03u MHz high precision timer.\n",
-		       ((mips_hpt_frequency + 500) / 1000) / 1000,
-		       ((mips_hpt_frequency + 500) / 1000) % 1000);
-		init_mips_clocksource();
+		/*
+		 * I don't have erratas for newer R4400 so be paranoid.
+		 */
+		return 1;
 	}
 
-	mips_clockevent_init();
+	return 0;
+}
+
+void __init time_init(void)
+{
+	plat_time_init();
+
+	if (mips_clockevent_init() || !cpu_has_mfc0_count_bug())
+		init_mips_clocksource();
 }

include/asm-mips/time.h

@@ -58,11 +58,12 @@ extern int (*perf_irq)(void);
  * Initialize the calling CPU's compare interrupt as clockevent device
  */
 #ifdef CONFIG_CEVT_R4K
-extern void mips_clockevent_init(void);
+extern int mips_clockevent_init(void);
 extern unsigned int __weak get_c0_compare_int(void);
 #else
-static inline void mips_clockevent_init(void)
+static inline int mips_clockevent_init(void)
 {
+	return -ENXIO;
 }
 #endif