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>

diff --git a/drivers/clocksource/timer-stm32.c b/drivers/clocksource/timer-stm32.c
index 33c7c90412ba31e29ca362b0352cd91f7d790af2..928ac281f937a8cac7811df9376d40912fed0728 100644 (file)
--- a/drivers/clocksource/timer-stm32.c
+++ b/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);