Shrink root dir size for tiny (<=128K) FAT12 fs

lib/oofatfs/ff.c

@@ -5422,7 +5422,8 @@ FRESULT f_mkfs (
 )
 {
     const UINT n_fats = 1;      /* Number of FATs for FAT/FAT32 volume (1 or 2) */
-    const UINT n_rootdir = 512; /* Number of root directory entries for FAT volume */
+    // CIRCUITPY-CHANGE: Make number of root directory entries changeable. See below.
+    UINT n_rootdir = 512;       /* Default number of root directory entries for FAT volume */
     static const WORD cst[] = {1, 4, 16, 64, 256, 512, 0};  /* Cluster size boundary for FAT volume (4Ks unit) */
 #if FF_MKFS_FAT32
     static const WORD cst32[] = {1, 2, 4, 8, 16, 32, 0};    /* Cluster size boundary for FAT32 volume (128Ks unit) */
@@ -5703,6 +5704,11 @@ FRESULT f_mkfs (
                 }
                 sz_fat = (n + ss - 1) / ss;     /* FAT size [sector] */
                 sz_rsv = 1;                     /* Number of reserved sectors */
+               // CIRCUITPY-CHANGE: For fewer than 256 clusters (128kB filesystem),
+               // shrink the root directory size from 512 entries to 128 entries. Note that
+               // long filenames will use two entries. This change affects only the root directory,
+               // not subdirectories
+                n_rootdir = (sz_vol <= 256) ? 128 : n_rootdir;
                 sz_dir = (DWORD)n_rootdir * SZDIRE / ss;    /* Rootdir size [sector] */
             }
             b_fat = b_vol + sz_rsv;                     /* FAT base */

tests/extmod/vfs_blockdev.py.exp

@@ -1,5 +1,5 @@
 test <class 'VfsFat'>
-(512, 512, 16, 16, 16, 0, 0, 0, 0, 255)
+(512, 512, 40, 40, 40, 0, 0, 0, 0, 255)
 [('test', 32768, 0, 90)]
 some datasome datasome datasome datasome datasome datasome datasome datasome datasome data
 test <class 'VfsLfs2'>

tests/extmod/vfs_fat_ramdisk.py.exp

@@ -1,7 +1,7 @@
 True
 True
 label: LABEL TEST
-statvfs: (512, 512, 16, 16, 16, 0, 0, 0, 0, 255)
+statvfs: (512, 512, 40, 40, 40, 0, 0, 0, 0, 255)
 getcwd: /
 True
 [('foo_file.txt', 32768, 0, 6)]