stm32: fix RTC rounding error breaking alarms

On STM32F4, when converting micro-seconds to the RTC sub-second counter
value, the current computation in the us_to_rtcss() routine has a large
rounding error which can even led to generate a negative value.
When such a negative value is output and then programmed in the
(unsigned) RTC_ALRMASSR register used to set the alarm precise
sub-second timestamp, it might put a wrong value in the past.
As a consequence when the RTC alarm is used a wake-up mechanism for the
low power idle, it might never fired and trigger a watchdog reboot.

An example of bad values on a STM32F412 with the RTC driven by the
32-kHz LSI:
- RTC_PREDIV_A = 1
- RTC_FREQ = (STM32F4_LSI_CLOCK / (RTC_PREDIV_A + 1) = 16000 /* Hz */
- RTC_PREDIV_S = (RTC_FREQ - 1) = 15999
- US_PER_RTC_TICK = 1000000 / RTC_FREQ = 62 /* rounded from 62.5 */
When converting 996000 us,
 us_to_rtcss(996000) = RTC_PREDIV_S - (us / US_PER_RTC_TICK)
                     = 15999 - (996000 / 62)
                     = -65
 returned as a uint32_t as 0xfffffffb.

Signed-off-by: Vincent Palatin <vpalatin@chromium.org>

BUG=b:130561737
TEST=manual, with the STOP mode enabled, we no longer see watchdog
reboot due to the RTC alarm being set in the past and never firing.
TEST=manual, verify that the output of the 'gettime' console command is not
drifting compared to the wall clock when the low power idle using the
RTC time is used.
BRANCH=fpmcu-bloonchipper

Change-Id: I53869539828bed9a5900d29407b5feba140b8217
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2563684
Tested-by: Vincent Palatin <vpalatin@chromium.org>
Reviewed-by: Nicolas Boichat <drinkcat@chromium.org>
Commit-Queue: Vincent Palatin <vpalatin@chromium.org>

1 files changed, 7 insertions, 3 deletions

diff --git a/chip/stm32/clock-stm32f4.c b/chip/stm32/clock-stm32f4.c
index 51d6027ef6..3580b7a02a 100644
--- a/chip/stm32/clock-stm32f4.c
+++ b/chip/stm32/clock-stm32f4.c
@@ -337,16 +337,20 @@ void clock_enable_module(enum module_id module, int enable)
 #define RTC_FREQ (STM32F4_LSI_CLOCK / (RTC_PREDIV_A + 1)) /* Hz */
 #endif
 #define RTC_PREDIV_S (RTC_FREQ - 1)
-#define US_PER_RTC_TICK (1000000 / RTC_FREQ)
+/*
+ * Scaling factor to ensure that the intermediate values computed from/to the
+ * RTC frequency are fitting in a 32-bit integer.
+ */
+#define SCALING 1000
 
 int32_t rtcss_to_us(uint32_t rtcss)
 {
-	return ((RTC_PREDIV_S - rtcss) * US_PER_RTC_TICK);
+	return ((RTC_PREDIV_S - rtcss) * (SECOND/SCALING) / (RTC_FREQ/SCALING));
 }
 
 uint32_t us_to_rtcss(int32_t us)
 {
-	return (RTC_PREDIV_S - (us / US_PER_RTC_TICK));
+	return (RTC_PREDIV_S - (us * (RTC_FREQ/SCALING) / (SECOND/SCALING)));
 }
 
 void rtc_init(void)