md: add a new enum type sync_action

In order to make code related to sync_thread cleaner in following
patches, also add detail comment about each sync action. And also
prepare to remove the related recovery_flags in the fulture.
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20240611132251.1967786-3-yukuai1@huaweicloud.com

drivers/md/md.h

@@ -34,6 +34,61 @@
  */
 #define	MD_FAILFAST	(REQ_FAILFAST_DEV | REQ_FAILFAST_TRANSPORT)
 
+/* Status of sync thread. */
+enum sync_action {
+	/*
+	 * Represent by MD_RECOVERY_SYNC, start when:
+	 * 1) after assemble, sync data from first rdev to other copies, this
+	 * must be done first before other sync actions and will only execute
+	 * once;
+	 * 2) resize the array(notice that this is not reshape), sync data for
+	 * the new range;
+	 */
+	ACTION_RESYNC,
+	/*
+	 * Represent by MD_RECOVERY_RECOVER, start when:
+	 * 1) for new replacement, sync data based on the replace rdev or
+	 * available copies from other rdev;
+	 * 2) for new member disk while the array is degraded, sync data from
+	 * other rdev;
+	 * 3) reassemble after power failure or re-add a hot removed rdev, sync
+	 * data from first rdev to other copies based on bitmap;
+	 */
+	ACTION_RECOVER,
+	/*
+	 * Represent by MD_RECOVERY_SYNC | MD_RECOVERY_REQUESTED |
+	 * MD_RECOVERY_CHECK, start when user echo "check" to sysfs api
+	 * sync_action, used to check if data copies from differenct rdev are
+	 * the same. The number of mismatch sectors will be exported to user
+	 * by sysfs api mismatch_cnt;
+	 */
+	ACTION_CHECK,
+	/*
+	 * Represent by MD_RECOVERY_SYNC | MD_RECOVERY_REQUESTED, start when
+	 * user echo "repair" to sysfs api sync_action, usually paired with
+	 * ACTION_CHECK, used to force syncing data once user found that there
+	 * are inconsistent data,
+	 */
+	ACTION_REPAIR,
+	/*
+	 * Represent by MD_RECOVERY_RESHAPE, start when new member disk is added
+	 * to the conf, notice that this is different from spares or
+	 * replacement;
+	 */
+	ACTION_RESHAPE,
+	/*
+	 * Represent by MD_RECOVERY_FROZEN, can be set by sysfs api sync_action
+	 * or internal usage like setting the array read-only, will forbid above
+	 * actions.
+	 */
+	ACTION_FROZEN,
+	/*
+	 * All above actions don't match.
+	 */
+	ACTION_IDLE,
+	NR_SYNC_ACTIONS,
+};
+
 /*
  * The struct embedded in rdev is used to serialize IO.
  */
@@ -571,7 +626,7 @@ enum recovery_flags {
 	/* interrupted because io-error */
 	MD_RECOVERY_ERROR,
 
-	/* flags determines sync action */
+	/* flags determines sync action, see details in enum sync_action */
 
 	/* if just this flag is set, action is resync. */
 	MD_RECOVERY_SYNC,