Import of the watch repository from Pebble

platform/silk/boot/src/util/delay.c

@@ -0,0 +1,49 @@
+/*
+ * Copyright 2024 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "delay.h"
+#include "stm32f4xx.h"
+#include "util/attributes.h"
+
+#include <inttypes.h>
+
+// Calculated using the formula from the firmware's delay_init:
+// ceil( NS_PER_US / ( clock_period * INSTRUCTIONS_PER_LOOP )
+// where NS_PER_US = 1000 and INSTRUCTIONS_PER_LOOP = 3,
+// and clock_period = 62.5
+#define LOOPS_PER_US (6)
+
+void NOINLINE delay_us(uint32_t us) {
+  uint32_t delay_loops = us * LOOPS_PER_US;
+
+  __asm volatile (
+      "spinloop:                             \n"
+      "  subs %[delay_loops], #1             \n"
+      "  bne spinloop                        \n"
+      : [delay_loops] "+r" (delay_loops) // read-write operand
+      :
+      : "cc"
+       );
+}
+
+void delay_ms(uint32_t millis) {
+  // delay_us(millis*1000) is not used because a long delay could easily
+  // overflow the veriable. Without the outer loop, a delay of even five
+  // seconds would overflow.
+  while (millis--) {
+    delay_us(1000);
+  }
+}