clocksource/drivers/arm_arch_timer: Correct fault programming of CNTKCTL_EL1.EVNTI

ARM virtual counter supports event stream, it can only trigger an event
when the trigger bit (the value of CNTKCTL_EL1.EVNTI) of CNTVCT_EL0 changes,
so the actual period of event stream is 2^(cntkctl_evnti + 1). For example,
when the trigger bit is 0, then virtual counter trigger an event for every
two cycles.

While we're at it, rework the way we compute the trigger bit position
by making it more obvious that when bits [n:n-1] are both set (with n
being the most significant bit), we pick bit (n + 1).

Fixes: 037f6377 ("drivers: clocksource: add support for ARM architected timer event stream")
Suggested-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Keqian Zhu <zhukeqian1@huawei.com>
Acked-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20201204073126.6920-3-zhukeqian1@huawei.com

drivers/clocksource/arm_arch_timer.c

@@ -822,15 +822,24 @@ static void arch_timer_evtstrm_enable(int divider)
 
 static void arch_timer_configure_evtstream(void)
 {
-	int evt_stream_div, pos;
+	int evt_stream_div, lsb;
+
+	/*
+	 * As the event stream can at most be generated at half the frequency
+	 * of the counter, use half the frequency when computing the divider.
+	 */
+	evt_stream_div = arch_timer_rate / ARCH_TIMER_EVT_STREAM_FREQ / 2;
+
+	/*
+	 * Find the closest power of two to the divisor. If the adjacent bit
+	 * of lsb (last set bit, starts from 0) is set, then we use (lsb + 1).
+	 */
+	lsb = fls(evt_stream_div) - 1;
+	if (lsb > 0 && (evt_stream_div & BIT(lsb - 1)))
+		lsb++;
 
-	/* Find the closest power of two to the divisor */
-	evt_stream_div = arch_timer_rate / ARCH_TIMER_EVT_STREAM_FREQ;
-	pos = fls(evt_stream_div);
-	if (pos > 1 && !(evt_stream_div & (1 << (pos - 2))))
-		pos--;
 	/* enable event stream */
-	arch_timer_evtstrm_enable(min(pos, 15));
+	arch_timer_evtstrm_enable(max(0, min(lsb, 15)));
 }
 
 static void arch_counter_set_user_access(void)