clocksource/drivers/stm32: Compute a prescaler value with a targeted rate

The prescaler value is arbitrarily set to 1024 without any regard to the
timer frequency. For 32-bit timers, there is no need to set a prescaler
value as they wrap in an acceptable interval and give the opportunity to
have precise timers on this platform. However, for 16-bit timers a prescaler
value is needed if we don't want to wrap too often per second which is
inefficient and adds more and more error margin. With a targeted clock
of 10MHz, the 16 bits are precise enough whatever the timer frequency is
as we will compute the prescaler.

Tested-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1515418139-23276-15-git-send-email-daniel.lezcano@linaro.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

drivers/clocksource/timer-stm32.c

@@ -37,6 +37,9 @@
 
 #define TIM_EGR_UG	BIT(0)
 
+#define TIM_PSC_MAX	USHRT_MAX
+#define TIM_PSC_CLKRATE	10000
+
 static int stm32_clock_event_shutdown(struct clock_event_device *clkevt)
 {
 	struct timer_of *to = to_timer_of(clkevt);
@@ -116,7 +119,14 @@ static void __init stm32_clockevent_init(struct timer_of *to)
 		prescaler = 1;
 		to->clkevt.rating = 250;
 	} else {
-		prescaler = 1024;
+		prescaler = DIV_ROUND_CLOSEST(timer_of_rate(to),
+					      TIM_PSC_CLKRATE);
+		/*
+		 * The prescaler register is an u16, the variable
+		 * can't be greater than TIM_PSC_MAX, let's cap it in
+		 * this case.
+		 */
+		prescaler = prescaler < TIM_PSC_MAX ? prescaler : TIM_PSC_MAX;
 		to->clkevt.rating = 100;
 	}
 	writel_relaxed(0, timer_of_base(to) + TIM_ARR);