sys: xtimer: add slow-timer-read compensation + test application

sys/include/xtimer.h

@@ -426,10 +426,34 @@ void _xtimer_sleep(uint32_t offset, uint32_t long_offset);
 static inline void xtimer_spin_until(uint32_t value);
 /** @} */
 
+#if XTIMER_MASK
+#ifndef XTIMER_SHIFT_ON_COMPARE
+/**
+ * @brief ignore some bits when comparing timer values
+ *
+ * (only relevant when XTIMER_MASK != 0, e.g., timers < 16bit.)
+ *
+ * When combining _xtimer_now() and _high_cnt, we have to get the same value in
+ * order to work around a race between overflowing _xtimer_now() and OR'ing the
+ * resulting values.
+ * But some platforms are too slow to get the same timer
+ * value twice, so we use this define to ignore some of the bits.
+ *
+ * Use tests/xtimer_shift_on_compare to find the correct value for your MCU.
+ */
+#define XTIMER_SHIFT_ON_COMPARE     0
+#endif
+#endif
+
 static inline uint32_t xtimer_now(void)
 {
 #if XTIMER_MASK
-    return _xtimer_now() | _high_cnt;
+    uint32_t a, b;
+    do {
+        a = _xtimer_now() | _high_cnt;
+        b = _xtimer_now() | _high_cnt;
+    } while ((a >> XTIMER_SHIFT_ON_COMPARE) != (b >> XTIMER_SHIFT_ON_COMPARE));
+    return b;
 #else
     return _xtimer_now();
 #endif

tests/xtimer_shift_on_compare/Makefile

@@ -0,0 +1,12 @@
+APPLICATION = xtimer_shift_on_compare
+include ../Makefile.tests_common
+
+BOARD_WHITELIST += msb-430h arduino-mega2560
+
+BOARD ?= native
+RIOTBASE ?= $(CURDIR)/../..
+
+FEATURES_REQUIRED += periph_timer
+USEMODULE += xtimer
+
+include $(RIOTBASE)/Makefile.include

tests/xtimer_shift_on_compare/main.c

@@ -0,0 +1,81 @@
+/*
+ * Copyright (C) 2015 Kaspar Schleiser <kaspar@schleiser.de>
+ *
+ * This file is subject to the terms and conditions of the GNU Lesser
+ * General Public License v2.1. See the file LICENSE in the top level
+ * directory for more details.
+ */
+
+/**
+ * @ingroup     tests
+ * @{
+ *
+ * @file
+ * @brief       timer test application
+ *
+ * @author      Kaspar Schleiser <kaspar@schleiser.de>
+ *
+ * @}
+ */
+
+#include <stdio.h>
+#include "xtimer.h"
+#include "periph_conf.h"
+
+#define ITERATIONS 128
+#define MAXSHIFT   8
+
+unsigned min[MAXSHIFT];
+unsigned max[MAXSHIFT];
+unsigned avg[MAXSHIFT];
+unsigned total[MAXSHIFT];
+
+int main(void)
+{
+    puts("xtimer_shift_on_compare test application.\n");
+
+    printf("trying %u iterations\n", ITERATIONS);
+
+    uint32_t a, b;
+    unsigned i = 0;
+
+    for (int shift = 0; shift < MAXSHIFT; shift++) {
+        min[shift] = (unsigned)0xffffffff;
+        max[shift] = 0;
+        total[shift] = 0;
+        avg[shift] = 0;
+
+        for (int j=0; j < ITERATIONS; j++) {
+            i = 0;
+            do {
+                i++;
+                if (i >= 0xF) {
+                    break;
+                }
+                else {
+                }
+
+                a = _xtimer_now() | _high_cnt;
+                b = _xtimer_now() | _high_cnt;
+            } while ((a>>shift) != (b>>shift));
+
+            min[shift] = i < min[shift] ? i : min[shift];
+            max[shift] = i > max[shift] ? i : max[shift];
+            if (i != 0xF) {
+                total[shift]++;
+                avg[shift] += i;
+            }
+        }
+    }
+
+    for (int i=0; i < MAXSHIFT; i++) {
+        if (min[i] >= 0xF) {
+            printf("shift %i is too little.\n", i);
+        } else {
+            printf("shift %i min=%u max=%u n_success=%u avg=%u\n", i, min[i], max[i], total[i], avg[i]/total[i]);
+        }
+    }
+
+    printf("\nTest complete.\n");
+    return 0;
+}