[sw] Collect various "runtime-ey" functions into lib/runtime.
This directory is made distinct from libbase mostly for semantic
reasons:
- It needs to know timing-ey things about Ibex, which are isolated in
ibex.h.
- It's more "system-aware" than what we tend to put in libbase.
hart.h contains "execution control" functions that may be
RISC-V-flavored, but independent of ibex.h.
Signed-off-by: Miguel Young de la Sota <mcyoung@google.com>
diff --git a/sw/device/lib/runtime/ibex.h b/sw/device/lib/runtime/ibex.h
new file mode 100644
index 0000000..a31d609
--- /dev/null
+++ b/sw/device/lib/runtime/ibex.h
@@ -0,0 +1,41 @@
+// Copyright lowRISC contributors.
+// Licensed under the Apache License, Version 2.0, see LICENSE for details.
+// SPDX-License-Identifier: Apache-2.0
+
+#ifndef SW_DEVICE_LIB_RUNTIME_IBEX_H_
+#define SW_DEVICE_LIB_RUNTIME_IBEX_H_
+
+#include <stddef.h>
+
+#include "sw/device/lib/base/stdasm.h"
+
+/**
+ * This header provides Ibex-specific functions, such as the clock frequency and
+ * cycle-accurate busy loops.
+ */
+
+/**
+ * The clock frequency of the Ibex core, in hertz.
+ */
+extern const size_t kIbexClockFreqHz;
+
+/**
+ * Spins for roughly the number of |cycles| given. For best results, |cycles|
+ * should be a multiple of eight.
+ *
+ * This function should not be used for time-keeping.
+ *
+ * @param cycles the number of cycles to burn.
+ */
+inline void ibex_busy_loop(size_t cycles) {
+ size_t out; // Used to create an inout parameter below.
+ asm volatile(
+ "busy_loop%=:"
+ " nop; nop; nop; nop;" // 4 cycles.
+ " addi %1, %1, -8;" // 1 cycle.
+ " blez %1, busy_loop%=;" // 3 cycles.
+ : "=&r"(out)
+ : "0"(cycles));
+}
+
+#endif // SW_DEVICE_LIB_RUNTIME_IBEX_H_
