taa_data->timer_irq_start_time = start;
taa_data->timer_irq_duration = duration;
- taa_data->timer_irq_start_delay = taa_data->timer_irq_start_time - expected_start;
+ /*
+ * We are dealing with two different clock sources: the
+ * external clock source that timerlat uses as a reference
+ * and the clock used by the tracer. There are also two
+ * moments: the time reading the clock and the timer in
+ * which the event is placed in the buffer (the trace
+ * event timestamp). If the processor is slow or there
+ * is some hardware noise, the difference between the
+ * timestamp and the external clock read can be longer
+ * than the IRQ handler delay, resulting in a negative
+ * time. If so, set IRQ start delay as 0. In the end,
+ * it is less relevant than the noise.
+ */
+ if (expected_start < taa_data->timer_irq_start_time)
+ taa_data->timer_irq_start_delay = taa_data->timer_irq_start_time - expected_start;
+ else
+ taa_data->timer_irq_start_delay = 0;
/*
* not exit from idle.