BACKPORT: tsc: provide monotonic timer

Implement the timer_monotonic_get() using the TSC.

Change-Id: I2c09ba2a2e96861255c7bf38291d9f4df004dcc4
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/49751
Reviewed-by: Duncan Laurie <dlaurie@chromium.org>

src/cpu/x86/Kconfig

@@ -25,6 +25,13 @@ config UDELAY_TSC
 	bool
 	default n
 
+config TSC_MONOTONIC_TIMER
+	def_bool n
+	depends on UDELAY_TSC
+	select HAVE_MONOTONIC_TIMER
+	help
+	  Expose monotonic time using the TSC.
+
 config UDELAY_TIMER2
 	bool
 	default n

src/cpu/x86/tsc/delay_tsc.c

@@ -164,3 +164,40 @@ void udelay(unsigned us)
 		count = rdtscll();
         }
 }
+
+#if CONFIG_TSC_MONOTONIC_TIMER
+#include <timer.h>
+
+static struct monotonic_counter {
+	int initialized;
+	struct mono_time time;
+	uint64_t last_value;
+} mono_counter;
+
+void timer_monotonic_get(struct mono_time *mt)
+{
+	uint64_t current_tick;
+	uint64_t ticks_elapsed;
+
+	if (!mono_counter.initialized) {
+		init_timer();
+		mono_counter.last_value = rdtscll();
+		mono_counter.initialized = 1;
+	}
+
+	current_tick = rdtscll();
+	ticks_elapsed = current_tick - mono_counter.last_value;
+
+	/* Update current time and tick values only if a full tick occurred. */
+	if (ticks_elapsed >= clocks_per_usec) {
+		uint64_t usecs_elapsed;
+
+		usecs_elapsed = ticks_elapsed / clocks_per_usec;
+		mono_time_add_usecs(&mono_counter.time, (long)usecs_elapsed);
+		mono_counter.last_value = current_tick;
+	}
+
+	/* Save result. */
+	*mt = mono_counter.time;
+}
+#endif