[PATCH 2/4] clocksource: stm32: use prescaler to adjust the resolution

From: Benjamin Gaignard
Date: Fri Dec 15 2017 - 03:53:16 EST


Rather than use fixed prescaler values compute it to get a clock
as close as possible of 10KHz and a resolution of 0.1ms.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@xxxxxx>
---
drivers/clocksource/timer-stm32.c | 23 ++++++++++++++++-------
1 file changed, 16 insertions(+), 7 deletions(-)

diff --git a/drivers/clocksource/timer-stm32.c b/drivers/clocksource/timer-stm32.c
index 23a321cca45b..de721d318065 100644
--- a/drivers/clocksource/timer-stm32.c
+++ b/drivers/clocksource/timer-stm32.c
@@ -37,6 +37,11 @@

#define TIM_EGR_UG BIT(0)

+#define MAX_TIM_PSC 0xFFFF
+
+/* Target a 10KHz clock to get a resolution of 0.1 ms */
+#define TARGETED_CLK_RATE 10000
+
static int stm32_clock_event_shutdown(struct clock_event_device *evt)
{
struct timer_of *to = to_timer_of(evt);
@@ -83,7 +88,7 @@ static irqreturn_t stm32_clock_event_handler(int irq, void *dev_id)
static void __init stm32_clockevent_init(struct timer_of *to)
{
unsigned long max_delta;
- int prescaler;
+ unsigned long prescaler;

to->clkevt.name = "stm32_clockevent";
to->clkevt.features = CLOCK_EVT_FEAT_PERIODIC;
@@ -96,13 +101,17 @@ static void __init stm32_clockevent_init(struct timer_of *to)
/* Detect whether the timer is 16 or 32 bits */
writel_relaxed(~0U, timer_of_base(to) + TIM_ARR);
max_delta = readl_relaxed(timer_of_base(to) + TIM_ARR);
- if (max_delta == ~0U) {
- prescaler = 1;
+ to->clkevt.rating = 50;
+ if (max_delta == ~0U)
to->clkevt.rating = 250;
- } else {
- prescaler = 1024;
- to->clkevt.rating = 50;
- }
+
+ /*
+ * Get the highest possible prescaler value to be as close
+ * as possible of TARGETED_CLK_RATE
+ */
+ prescaler = DIV_ROUND_CLOSEST(timer_of_rate(to), TARGETED_CLK_RATE);
+ if (prescaler > MAX_TIM_PSC)
+ prescaler = MAX_TIM_PSC;

writel_relaxed(0, timer_of_base(to) + TIM_ARR);
writel_relaxed(prescaler - 1, timer_of_base(to) + TIM_PSC);
--
2.15.0