Use flash_safe_execute

Although it's not required in this example, demonstrate how to call
flash function using flash_safe_execute

Fixes raspberrypi#510

flash/program/flash_program.c

@@ -8,6 +8,7 @@
 #include <stdlib.h>
 
 #include "pico/stdlib.h"
+#include "pico/flash.h"
 #include "hardware/flash.h"
 
 // We're going to erase and reprogram a region 256k from the start of flash.
@@ -26,6 +27,19 @@ void print_buf(const uint8_t *buf, size_t len) {
     }
 }
 
+// This function will be called when it's safe to call flash_range_erase
+static void call_flash_range_erase(void *param) {
+    uint32_t offset = (uint32_t)param;
+    flash_range_erase(offset, FLASH_SECTOR_SIZE);
+}
+
+// This function will be called when it's safe to call flash_range_program
+static void call_flash_range_program(void *param) {
+    uint32_t offset = ((uintptr_t*)param)[0];
+    const uint8_t *data = (const uint8_t *)((uintptr_t*)param)[1];
+    flash_range_program(offset, data, FLASH_PAGE_SIZE);
+}
+
 int main() {
     stdio_init_all();
     uint8_t random_data[FLASH_PAGE_SIZE];
@@ -37,12 +51,21 @@ int main() {
 
     // Note that a whole number of sectors must be erased at a time.
     printf("\nErasing target region...\n");
-    flash_range_erase(FLASH_TARGET_OFFSET, FLASH_SECTOR_SIZE);
+
+    // Flash is "execute in place" and so will be in use when any code runs, e.g. an interrupt handler or code running on a different core
+    // Calling flash_range_erase or flash_range_program at the same time as flash is running code would cause a crash.
+    // flash_safe_execute disables interrupts and tries to cooperate with the other core to ensure flash is not in use
+    // See the documentation for flash_safe_execute and its assumptions and limitations
+    int rc = flash_safe_execute(call_flash_range_erase, (void*)FLASH_TARGET_OFFSET, UINT32_MAX);
+    hard_assert(rc == PICO_OK);
+
     printf("Done. Read back target region:\n");
     print_buf(flash_target_contents, FLASH_PAGE_SIZE);
 
     printf("\nProgramming target region...\n");
-    flash_range_program(FLASH_TARGET_OFFSET, random_data, FLASH_PAGE_SIZE);
+    uintptr_t params[] = { FLASH_TARGET_OFFSET, (uintptr_t)random_data};
+    rc = flash_safe_execute(call_flash_range_program, params, UINT32_MAX);
+    hard_assert(rc == PICO_OK);
     printf("Done. Read back target region:\n");
     print_buf(flash_target_contents, FLASH_PAGE_SIZE);