From efbd5518f6677feae5b454f1a6f4e93f0dbec5f2 Mon Sep 17 00:00:00 2001
From: Kaspar Schleiser <kaspar@schleiser.de>
Date: Sat, 5 Sep 2015 01:18:36 +0200
Subject: [PATCH] cpu: stm32f1: timer: fix race when combining two 16bit timer
values
---
cpu/stm32f1/periph/timer.c | 25 +++++++++++++++++++++----
1 file changed, 21 insertions(+), 4 deletions(-)
diff --git a/cpu/stm32f1/periph/timer.c b/cpu/stm32f1/periph/timer.c
index d2ce653bd2..0513d0350c 100644
--- a/cpu/stm32f1/periph/timer.c
+++ b/cpu/stm32f1/periph/timer.c
@@ -220,16 +220,33 @@ int timer_clear(tim_t dev, int channel)
unsigned int timer_read(tim_t dev)
{
+ unsigned a, b;
switch (dev) {
#if TIMER_0_EN
case TIMER_0:
- return (((unsigned int)(0xffff & TIMER_0_DEV_0->CNT)) | (TIMER_0_DEV_1->CNT<<16));
- break;
+ /* do OR'ing two times and only use value if results are equal.
+ * otherwise, the lower 16bit counter could overflow while the
+ * upper counter is read, leading to an incorrect result. */
+ do {
+ a = (((unsigned int)(0xffff & TIMER_0_DEV_0->CNT)) |
+ (TIMER_0_DEV_1->CNT<<16));
+ b = (((unsigned int)(0xffff & TIMER_0_DEV_0->CNT)) |
+ (TIMER_0_DEV_1->CNT<<16));
+ } while (a!=b);
+
+ return a;
#endif
#if TIMER_1_EN
case TIMER_1:
- return (((unsigned int)(0xffff & TIMER_1_DEV_0->CNT)) | (TIMER_1_DEV_1->CNT<<16));
- break;
+ /* see above about why loop is needed */
+ do {
+ a = (((unsigned int)(0xffff & TIMER_1_DEV_0->CNT)) |
+ (TIMER_1_DEV_1->CNT<<16));
+ b = (((unsigned int)(0xffff & TIMER_1_DEV_0->CNT)) |
+ (TIMER_1_DEV_1->CNT<<16));
+ } while (a!=b);
+
+ return a;
#endif
case TIMER_UNDEFINED:
default:
--
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