loader: Add boot hook for slot selection

boot/bootutil/include/bootutil/boot_hooks.h

@@ -82,6 +82,18 @@
 
 #endif /* MCUBOOT_BOOT_GO_HOOKS  */
 
+#ifdef MCUBOOT_FIND_NEXT_SLOT_HOOKS
+
+#define BOOT_HOOK_FIND_SLOT_CALL(f, ret_default, ...) \
+    DO_HOOK_CALL(f, ret_default, __VA_ARGS__)
+
+#else
+
+#define BOOT_HOOK_FIND_SLOT_CALL(f, ret_default, ...) \
+    HOOK_CALL_NOP(f, ret_default, __VA_ARGS__)
+
+#endif /* MCUBOOT_FIND_NEXT_SLOT_HOOKS */
+
 #ifdef MCUBOOT_FLASH_AREA_HOOKS
 
 #define BOOT_HOOK_FLASH_AREA_CALL(f, ret_default, ...) \
@@ -260,4 +272,16 @@ int flash_area_get_device_id_hook(const struct flash_area *fa,
 #define BOOT_RESET_REQUEST_HOOK_CHECK_FAILED	3
 #define BOOT_RESET_REQUEST_HOOK_INTERNAL_ERROR	4
 
+/**
+ * Finds the preferred slot containing the image.
+ *
+ * @param[in]   state        Boot loader status information.
+ * @param[in]   image        Image, for which the slot should be found.
+ * @param[out]  active_slot  Number of the preferred slot.
+ *
+ * @return 0 if a slot was requested;
+ *         BOOT_HOOK_REGULAR follow the normal execution path.
+ */
+int boot_find_next_slot_hook(struct boot_loader_state *state, uint8_t image, uint32_t *active_slot);
+
 #endif /*H_BOOTUTIL_HOOKS*/

boot/bootutil/src/loader.c

@@ -2818,7 +2818,12 @@ boot_load_and_validate_images(struct boot_loader_state *state)
                 break;
             }
 
-            active_slot = find_slot_with_highest_version(state);
+            rc = BOOT_HOOK_FIND_SLOT_CALL(boot_find_next_slot_hook, BOOT_HOOK_REGULAR,
+                                          state, BOOT_CURR_IMG(state), &active_slot);
+            if (rc == BOOT_HOOK_REGULAR) {
+                active_slot = find_slot_with_highest_version(state);
+            }
+
             if (active_slot == NO_ACTIVE_SLOT) {
                 BOOT_LOG_INF("No slot to load for image %d",
                              BOOT_CURR_IMG(state));
@@ -2865,7 +2870,12 @@ boot_load_and_validate_images(struct boot_loader_state *state)
                 break;
             }
 
-            active_slot = find_slot_with_highest_version(state);
+            rc = BOOT_HOOK_FIND_SLOT_CALL(boot_find_next_slot_hook, BOOT_HOOK_REGULAR,
+                                          state, BOOT_CURR_IMG(state), &active_slot);
+            if (rc == BOOT_HOOK_REGULAR) {
+                active_slot = find_slot_with_highest_version(state);
+            }
+
             if (active_slot == NO_ACTIVE_SLOT) {
                 BOOT_LOG_INF("No slot to load for image %d",
                              BOOT_CURR_IMG(state));

boot/zephyr/Kconfig

@@ -1096,6 +1096,13 @@ config MCUBOOT_ACTION_HOOKS
 	  'mcuboot_status_type_t' is listed in
 	  boot/bootutil/include/bootutil/mcuboot_status.h
 
+config FIND_NEXT_SLOT_HOOKS
+	bool "Enable hooks for finding the next active slot"
+	help
+	  Allow to provide procedures for override or extend the search policy
+	  for the best slot to boot in the Direct XIP mode.
+	  By default a slot with the highest version is selected.
+
 config BOOT_DISABLE_CACHES
 	bool "Disable I/D caches before chain-loading application"
 	depends on CPU_HAS_ICACHE || CPU_HAS_DCACHE

boot/zephyr/hooks_sample.c

@@ -93,3 +93,8 @@ int boot_img_install_stat_hook(int image_index, int slot, int *img_install_stat)
 {
     return BOOT_HOOK_REGULAR;
 }
+
+int boot_find_next_slot_hook(struct boot_loader_state *state, uint8_t image, uint32_t *active_slot)
+{
+    return BOOT_HOOK_REGULAR;
+}

boot/zephyr/include/mcuboot_config/mcuboot_config.h

@@ -278,6 +278,10 @@
 #define MCUBOOT_FLASH_AREA_HOOKS
 #endif
 
+#ifdef CONFIG_FIND_NEXT_SLOT_HOOKS
+#define MCUBOOT_FIND_NEXT_SLOT_HOOKS
+#endif
+
 #ifdef CONFIG_MCUBOOT_VERIFY_IMG_ADDRESS
 #define MCUBOOT_VERIFY_IMG_ADDRESS
 #endif