gpu: nvgpu: add nvgpu_current_time_ns()

Add an abstraction over a monotonic system clock in nanosecond units.
Use ktime_get() for the Linux implementation, similarly to
nvgpu_current_time_ms().

Jira NVGPU-708

Change-Id: I3165c20abf2652f1a1fa04e66c04cd34a8fe6dcc
Signed-off-by: Konsta Holtta <kholtta@nvidia.com>
Reviewed-on: https://git-master.nvidia.com/r/1723201
Reviewed-by: svc-mobile-coverity <svc-mobile-coverity@nvidia.com>
Reviewed-by: Deepak Nibade <dnibade@nvidia.com>
Reviewed-by: Sourab Gupta <sourabg@nvidia.com>
Reviewed-by: Alex Waterman <alexw@nvidia.com>
GVS: Gerrit_Virtual_Submit
Reviewed-by: Terje Bergstrom <tbergstrom@nvidia.com>
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Tested-by: mobile promotions <svcmobile_promotions@nvidia.com>

drivers/gpu/nvgpu/common/linux/timers.c

@@ -242,6 +242,18 @@ s64 nvgpu_current_time_ms(void)
 	return ktime_to_ms(ktime_get());
 }
 
+/**
+ * nvgpu_current_time_ns - Time in nanoseconds from a monotonic clock.
+ *
+ * Return a clock in nanosecond units. The start time of the clock is
+ * unspecified; the time returned can be compared with older ones to measure
+ * durations. The source clock does not jump when the system clock is adjusted.
+ */
+s64 nvgpu_current_time_ns(void)
+{
+	return ktime_to_ns(ktime_get());
+}
+
 /**
  * nvgpu_hr_timestamp - Opaque 'high resolution' time stamp.
  *