mips: csrc-r4k: Mark R4K timer as unstable if CPU freq changes

Commit 07d69579e7 ("MIPS: Don't register r4k sched clock when CPUFREQ
enabled") disabled the r4k-clock usage for scheduler ticks counting due
to the scheduler being non-tolerant for unstable clocks sources. For the
same reason the clock should be used in the system clocksource framework
with care. As soon as CPU frequency changes the clocksource framework
should be notified about this by marking the R4K timer being unstable
(which it really is, since the ticks rate has been changed synchronously
with the CPU frequency).

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Cc: Alexey Malahov <Alexey.Malahov@baikalelectronics.ru>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Paul Burton <paulburton@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: devicetree@vger.kernel.org
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/Kconfig

@@ -1108,6 +1108,7 @@ config CSRC_IOASIC
 	bool
 
 config CSRC_R4K
+	select CLOCKSOURCE_WATCHDOG if CPU_FREQ
 	bool
 
 config CSRC_SB1250

arch/mips/kernel/csrc-r4k.c

@@ -6,6 +6,7 @@
  * Copyright (C) 2007 by Ralf Baechle
  */
 #include <linux/clocksource.h>
+#include <linux/cpufreq.h>
 #include <linux/init.h>
 #include <linux/sched_clock.h>
 
@@ -65,6 +66,45 @@ static bool rdhwr_count_usable(void)
 	return false;
 }
 
+#ifdef CONFIG_CPU_FREQ
+
+static bool __read_mostly r4k_clock_unstable;
+
+static void r4k_clocksource_unstable(char *reason)
+{
+	if (r4k_clock_unstable)
+		return;
+
+	r4k_clock_unstable = true;
+
+	pr_info("R4K timer is unstable due to %s\n", reason);
+
+	clocksource_mark_unstable(&clocksource_mips);
+}
+
+static int r4k_cpufreq_callback(struct notifier_block *nb,
+				unsigned long val, void *data)
+{
+	if (val == CPUFREQ_POSTCHANGE)
+		r4k_clocksource_unstable("CPU frequency change");
+
+	return 0;
+}
+
+static struct notifier_block r4k_cpufreq_notifier = {
+	.notifier_call  = r4k_cpufreq_callback,
+};
+
+static int __init r4k_register_cpufreq_notifier(void)
+{
+	return cpufreq_register_notifier(&r4k_cpufreq_notifier,
+					 CPUFREQ_TRANSITION_NOTIFIER);
+
+}
+core_initcall(r4k_register_cpufreq_notifier);
+
+#endif /* !CONFIG_CPU_FREQ */
+
 int __init init_r4k_clocksource(void)
 {
 	if (!cpu_has_counter || !mips_hpt_frequency)