dosfs: Lazy update of FAT32 FS info sector

The FAT32 FS info sector contains hints for the free cluster count and
the next free cluster.  The previous code read these values during mount
and replaced them with invalid values.  The shutdown operation updated
them with the current values.  These values are only hints.  Every FAT
implementation must cope with arbitrary values.  They are intended to
speed up certain operations.

Now we update the free cluster count and next free culster in the FAT32
FS info sector only during unmount or sync operations and only if the
values have changed.  This avoids writes to the FS info sector and
conforms to the behaviour of Linux and Windows.

The application can force an update of these values now with the fsync()
and fdatasync() operations.  Applications that only read will perform
not write operations to the FAT32 FS info sector.

The new fat_sync() function performs all non-file specific
synchronizations.

1 files changed, 77 insertions, 64 deletions

diff --git a/cpukit/libfs/src/dosfs/fat.c b/cpukit/libfs/src/dosfs/fat.c
index d40aaf04c5..c2c3e89b26 100644
--- a/cpukit/libfs/src/dosfs/fat.c
+++ b/cpukit/libfs/src/dosfs/fat.c
@@ -574,16 +574,12 @@ fat_init_volume_info(fat_fs_info_t *fs_info, const char *device)
                     return -1;
                 }
 
-                vol->free_cls = FAT_GET_FSINFO_FREE_CLUSTER_COUNT(fs_info_sector);
-                vol->next_cl = FAT_GET_FSINFO_NEXT_FREE_CLUSTER(fs_info_sector);
-                rc = fat_fat32_update_fsinfo_sector(fs_info, FAT_UNDEFINED_VALUE,
-                                                    FAT_UNDEFINED_VALUE);
-                if ( rc != RC_OK )
-                {
-                    _fat_block_release(fs_info);
-                    close(vol->fd);
-                    return rc;
-                }
+                vol->free_cls_in_fs_info =
+                  FAT_GET_FSINFO_FREE_CLUSTER_COUNT(fs_info_sector);
+                vol->free_cls = vol->free_cls_in_fs_info;
+                vol->next_cl_in_fs_info =
+                  FAT_GET_FSINFO_NEXT_FREE_CLUSTER(fs_info_sector);
+                vol->next_cl = vol->next_cl_in_fs_info;
             }
         }
     }
@@ -645,6 +641,75 @@ fat_init_volume_info(fat_fs_info_t *fs_info, const char *device)
     return RC_OK;
 }
 
+/* fat_fat32_update_fsinfo_sector --
+ *     Synchronize fsinfo sector for FAT32 volumes
+ *
+ * PARAMETERS:
+ *     fs_info    - FS info
+ *
+ * RETURNS:
+ *     RC_OK on success, or -1 if error occured (errno set appropriately)
+ */
+static int
+fat_fat32_update_fsinfo_sector(fat_fs_info_t *fs_info)
+{
+    ssize_t ret1 = 0, ret2 = 0;
+
+    if (fs_info->vol.type == FAT_FAT32)
+    {
+        uint32_t free_count = fs_info->vol.free_cls;
+        uint32_t next_free = fs_info->vol.next_cl;
+
+        if (free_count != fs_info->vol.free_cls_in_fs_info)
+        {
+            uint32_t le_free_count = CT_LE_L(free_count);
+
+            fs_info->vol.free_cls_in_fs_info = free_count;
+
+            ret1 = _fat_block_write(fs_info,
+                                    fs_info->vol.info_sec,
+                                    FAT_FSINFO_FREE_CLUSTER_COUNT_OFFSET,
+                                    sizeof(le_free_count),
+                                    &le_free_count);
+        }
+
+        if (next_free != fs_info->vol.next_cl_in_fs_info)
+        {
+            uint32_t le_next_free = CT_LE_L(next_free);
+
+            fs_info->vol.next_cl_in_fs_info = next_free;
+
+            ret2 = _fat_block_write(fs_info,
+                                    fs_info->vol.info_sec,
+                                    FAT_FSINFO_NEXT_FREE_CLUSTER_OFFSET,
+                                    sizeof(le_next_free),
+                                    &le_next_free);
+        }
+    }
+
+    if ( (ret1 < 0) || (ret2 < 0) )
+        return -1;
+
+    return RC_OK;
+}
+
+int
+fat_sync(fat_fs_info_t *fs_info)
+{
+    int rc = RC_OK;
+
+    rc = fat_fat32_update_fsinfo_sector(fs_info);
+    if ( rc != RC_OK )
+        rc = -1;
+
+    fat_buf_release(fs_info);
+
+    if (rtems_bdbuf_syncdev(fs_info->vol.dd) != RTEMS_SUCCESSFUL)
+        rc = -1;
+
+    return rc;
+}
+
 /* fat_shutdown_drive --
  *     Free all allocated resources and synchronize all necessary data
  *
@@ -661,17 +726,8 @@ fat_shutdown_drive(fat_fs_info_t *fs_info)
     int            rc = RC_OK;
     int            i = 0;
 
-    if (fs_info->vol.type & FAT_FAT32)
-    {
-        rc = fat_fat32_update_fsinfo_sector(fs_info, fs_info->vol.free_cls,
-                                            fs_info->vol.next_cl);
-        if ( rc != RC_OK )
-            rc = -1;
-    }
-
-    fat_buf_release(fs_info);
-
-    if (rtems_bdbuf_syncdev(fs_info->vol.dd) != RTEMS_SUCCESSFUL)
+    rc = fat_sync(fs_info);
+    if ( rc != RC_OK )
         rc = -1;
 
     for (i = 0; i < FAT_HASH_SIZE; i++)
@@ -849,46 +905,3 @@ fat_ino_is_unique(
 
     return (ino >= fs_info->uino_base);
 }
-
-/* fat_fat32_update_fsinfo_sector --
- *     Synchronize fsinfo sector for FAT32 volumes
- *
- * PARAMETERS:
- *     fs_info    - FS info
- *     free_count - count of free clusters
- *     next_free  - the next free cluster num
- *
- * RETURNS:
- *     RC_OK on success, or -1 if error occured (errno set appropriately)
- */
-int
-fat_fat32_update_fsinfo_sector(
-    fat_fs_info_t                        *fs_info,
-    uint32_t                              free_count,
-    uint32_t                              next_free
-    )
-{
-    ssize_t                 ret1 = 0, ret2 = 0;
-    uint32_t                le_free_count = 0;
-    uint32_t                le_next_free = 0;
-
-    le_free_count = CT_LE_L(free_count);
-    le_next_free = CT_LE_L(next_free);
-
-    ret1 = _fat_block_write(fs_info,
-                            fs_info->vol.info_sec,
-                            FAT_FSINFO_FREE_CLUSTER_COUNT_OFFSET,
-                            4,
-                            (char *)(&le_free_count));
-
-    ret2 = _fat_block_write(fs_info,
-                            fs_info->vol.info_sec,
-                            FAT_FSINFO_NEXT_FREE_CLUSTER_OFFSET,
-                            4,
-                            (char *)(&le_next_free));
-
-    if ( (ret1 < 0) || (ret2 < 0) )
-        return -1;
-
-    return RC_OK;
-}