kern/clock: new module

1 files changed, 123 insertions, 0 deletions

diff --git a/kern/clock.h b/kern/clock.h
new file mode 100644
index 00000000..30db0a82
--- /dev/null
+++ b/kern/clock.h
@@ -0,0 +1,123 @@
+/*
+ * Copyright (c) 2017 Richard Braun.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ *
+ * Timekeeping module.
+ */
+
+#ifndef _KERN_CLOCK_H
+#define _KERN_CLOCK_H
+
+#include <stdbool.h>
+#include <stdint.h>
+
+#include <kern/atomic.h>
+#include <kern/clock_i.h>
+#include <kern/init.h>
+#include <kern/macros.h>
+
+/*
+ * Clock frequency.
+ *
+ * TODO Clock frequency selection.
+ */
+#define CLOCK_FREQ 200
+
+#if (1000 % CLOCK_FREQ) != 0
+#error "invalid clock frequency"
+#endif /* (1000 % CLOCK_FREQ) != 0 */
+
+/*
+ * Arbitrary value used to determine if a time is in the past or the future.
+ *
+ * Time is represented as 64-bits unsigned integers counting ticks. The
+ * global time currently starts from 0 but this isn't a strong assumption
+ * users should rely on. Instead, all time checks involve a time reference
+ * against which to compare. The result of that comparison, done by
+ * substraction, is either in the future, i.e. the difference is less
+ * than the expire threshold, or in the past, i.e. the difference is
+ * greater (keep in mind the result is unsigned). The threshold must be
+ * large enough to allow both a wide range of possible times in the future,
+ * but also enough time in the past for reliable timeout detection. Note
+ * that using signed integers would be equivalent to dividing the range
+ * in two (almost) equal past and future halves.
+ */
+#define CLOCK_EXPIRE_THRESHOLD (-(1ULL << 60))
+
+static inline uint64_t
+clock_get_time(void)
+{
+    extern union clock_global_time clock_global_time;
+
+#ifdef ATOMIC_HAVE_64B_OPS
+
+    /*
+     * Don't enforce a stronger memory order, since :
+     *  1/ it's useless as long as the reader remains on the same processor
+     *  2/ thread migration enforces sequential consistency
+     */
+    return atomic_load(&clock_global_time.ticks, ATOMIC_RELAXED);
+
+#else /* ATOMIC_HAVE_64B_OPS */
+
+    uint32_t high1, low, high2;
+
+    /*
+     * For machines with no 64-bits atomic accessors, this implementation uses
+     * a variant of the two-digit monotonic-clock algorithm, described in the
+     * paper "Concurrent Reading and Writing of Clocks" by Leslie Lamport.
+     */
+
+    do {
+        high1 = atomic_load_acquire(&clock_global_time.high1);
+        low = atomic_load_acquire(&clock_global_time.low);
+        high2 = atomic_load(&clock_global_time.high2, ATOMIC_RELAXED);
+    } while (high1 != high2);
+
+    return ((uint64_t)high2 << 32) | low;
+
+#endif /* ATOMIC_HAVE_64B_OPS */
+}
+
+static inline uint64_t
+clock_ticks_to_ms(uint64_t ticks)
+{
+    return ticks * (1000 / CLOCK_FREQ);
+}
+
+static inline uint64_t
+clock_ticks_from_ms(uint64_t ms)
+{
+    return DIV_CEIL(ms, (1000 / CLOCK_FREQ));
+}
+
+static inline bool
+clock_time_expired(uint64_t t, uint64_t ref)
+{
+    return (t - ref) > CLOCK_EXPIRE_THRESHOLD;
+}
+
+static inline bool
+clock_time_occurred(uint64_t t, uint64_t ref)
+{
+    return (t == ref) || clock_time_expired(t, ref);
+}
+
+void clock_tick_intr(void);
+
+INIT_OP_DECLARE(clock_setup);
+
+#endif /* _KERN_CLOCK_H */