tests/xtimer_drift: Introduce drift test application.

The xtimer_drift test application can be used to measure the drift of
the xtimer subsystem when referenced against the wall clock or,
preferably, a PC with NTP synchronized clock as reference.

tests/xtimer_drift/Makefile

+APPLICATION = xtimer_drift
+include ../Makefile.tests_common
+
+FEATURES_REQUIRED += periph_timer
+USEMODULE += xtimer
+
+include $(RIOTBASE)/Makefile.include

tests/xtimer_drift/main.c

+/*
+ * Copyright (C) 2015 Kaspar Schleiser <kaspar@schleiser.de>
+ * Copyright (C) 2015 Eistec AB
+ *               2013 INRIA
+ *
+ * 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    xtimer_drift test application
+ *
+ * @author   Kaspar Schleiser <kaspar@schleiser.de>
+ * @author   Oliver Hahm <oliver.hahm@inria.fr>
+ * @author   Christian Mehlis <mehlis@inf.fu-berlin.de>
+ * @author   Joakim Nohlgård <joakim.nohlgard@eistec.se>
+ *
+ * @}
+ */
+
+#include <stdio.h>
+#include <time.h>
+
+#include "xtimer.h"
+#include "thread.h"
+#include "msg.h"
+
+/* We generate some context switching and IPC traffic by using multiple threads
+ * and generate some xtimer load by scheduling several messages to be called at
+ * different times. TEST_HZ is the frequency of messages being sent from the
+ * main thread to the worker, all other message frequencies are derived from
+ * TEST_HZ.
+ * TEST_MSG_RX_USLEEP is a tiny sleep inside the message reception thread to
+ * cause extra context switches.
+ */
+#define TEST_HZ (64)
+#define TEST_INTERVAL (1000000 / TEST_HZ)
+#define TEST_MSG_RX_USLEEP (200)
+
+char slacker_stack1[THREAD_STACKSIZE_DEFAULT];
+char slacker_stack2[THREAD_STACKSIZE_DEFAULT];
+char worker_stack[THREAD_STACKSIZE_MAIN];
+
+struct timer_msg {
+    xtimer_t timer;
+    uint32_t interval;
+    msg_t msg;
+};
+
+struct timer_msg msg_a = { .interval = (TEST_INTERVAL / 2) };
+struct timer_msg msg_b = { .interval = (TEST_INTERVAL / 3) };
+struct timer_msg msg_c = { .interval = (TEST_INTERVAL * 5) };
+struct timer_msg msg_d = { .interval = (TEST_INTERVAL * 2) };
+
+/* This thread is only here to give the kernel some extra load */
+void *slacker_thread(void *arg)
+{
+    (void) arg;
+    timex_t now;
+
+    printf("Starting thread %" PRIkernel_pid "\n", thread_getpid());
+
+    /* we need a queue if the second message arrives while the first is still processed */
+    /* without a queue, the message would get lost */
+    msg_t msgq[4];
+    msg_init_queue(msgq, 4);
+
+    while (1) {
+        msg_t m;
+        msg_receive(&m);
+        struct timer_msg *tmsg = (struct timer_msg *) m.content.ptr;
+        xtimer_now_timex(&now);
+        xtimer_usleep(TEST_MSG_RX_USLEEP);
+
+        tmsg->msg.type = 12345;
+        tmsg->msg.content.ptr = (void*)tmsg;
+        xtimer_set_msg(&tmsg->timer, tmsg->interval, &tmsg->msg, thread_getpid());
+    }
+}
+
+/* This thread will print the drift to stdout once per second */
+void *worker_thread(void *arg)
+{
+    (void) arg;
+    unsigned int loop_counter = 0;
+    uint32_t start = 0;
+
+    printf("Starting thread %" PRIkernel_pid "\n", thread_getpid());
+
+    while (1) {
+        msg_t m;
+        msg_receive(&m);
+        uint32_t now = xtimer_now();
+        if (start == 0) {
+            start = now;
+        }
+
+        uint32_t us, sec;
+        unsigned int min, hr;
+        us = now % 1000000;
+        sec = now / 1000000;
+        min = (sec / 60) % 60;
+        hr = sec / 3600;
+        if ((loop_counter % TEST_HZ) == 0) {
+            uint32_t expected = start + loop_counter * TEST_INTERVAL;
+            int32_t diff = now - expected;
+            printf("now=%" PRIu32 ".%06" PRIu32 " (%u hours %u min), diff=%" PRId32 "\n",
+                sec, us, hr, min, diff);
+        }
+        ++loop_counter;
+    }
+}
+
+int main(void)
+{
+    msg_t m;
+
+    puts("xtimer_drift test application");
+    puts("Make note of the PC clock when starting this test, let run for a while, "
+        "compare the printed time against the expected time from the PC clock.");
+    puts("The difference is the RIOT timer drift, this is likely caused by either: "
+        "Inaccurate hardware timer, or bugs in the software (xtimer or periph/timer).");
+    printf("This test will run a periodic timer every %lu microseconds (%lu Hz), ",
+        (unsigned long)TEST_INTERVAL, (unsigned long)TEST_HZ);
+    puts("The current time will be printed once per second, along with the "
+         "difference between the actual and expected xtimer_now value.");
+    puts("Two other threads are also running only to cause extra interrupts and context switches.");
+    puts(" <====== PC clock if running in pyterm.");
+    puts("");
+    puts(" =======================");
+    puts(" ===== Test begins =====");
+    puts(" =======================");
+
+    kernel_pid_t pid1 = thread_create(
+        slacker_stack1,
+        sizeof(slacker_stack1),
+        THREAD_PRIORITY_MAIN - 1,
+        CREATE_STACKTEST,
+        slacker_thread,
+        NULL,
+        "slacker1");
+
+    puts("sending 1st msg");
+    m.content.ptr = (char *) &msg_a;
+    msg_try_send(&m, pid1);
+
+    puts("sending 2nd msg");
+    m.content.ptr = (char *) &msg_b;
+    msg_try_send(&m, pid1);
+
+    kernel_pid_t pid2 = thread_create(
+        slacker_stack2,
+        sizeof(slacker_stack2),
+        THREAD_PRIORITY_MAIN - 1,
+        CREATE_STACKTEST,
+        slacker_thread,
+        NULL,
+        "slacker2");
+
+    puts("sending 3rd msg");
+    m.content.ptr = (char *) &msg_c;
+    msg_try_send(&m, pid2);
+
+    puts("sending 4th msg");
+    m.content.ptr = (char *) &msg_d;
+    msg_try_send(&m, pid2);
+
+    kernel_pid_t pid3 = thread_create(
+                   worker_stack,
+                   sizeof(worker_stack),
+                   THREAD_PRIORITY_MAIN - 1,
+                   CREATE_STACKTEST,
+                   worker_thread,
+                   NULL,
+                   "worker");
+
+    uint32_t last_wakeup = xtimer_now();
+    while (1) {
+        xtimer_usleep_until(&last_wakeup, TEST_INTERVAL);
+        msg_try_send(&m, pid3);
+    }
+}