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1.. SPDX-License-Identifier: GPL-2.02 3================================4Optimized MPEG Filesystem (OMFS)5================================6 7Overview8========9 10OMFS is a filesystem created by SonicBlue for use in the ReplayTV DVR11and Rio Karma MP3 player. The filesystem is extent-based, utilizing12block sizes from 2k to 8k, with hash-based directories. This13filesystem driver may be used to read and write disks from these14devices.15 16Note, it is not recommended that this FS be used in place of a general17filesystem for your own streaming media device. Native Linux filesystems18will likely perform better.19 20More information is available at:21 22 http://linux-karma.sf.net/23 24Various utilities, including mkomfs and omfsck, are included with25omfsprogs, available at:26 27 https://bobcopeland.com/karma/28 29Instructions are included in its README.30 31Options32=======33 34OMFS supports the following mount-time options:35 36 ============ ========================================37 uid=n make all files owned by specified user38 gid=n make all files owned by specified group39 umask=xxx set permission umask to xxx40 fmask=xxx set umask to xxx for files41 dmask=xxx set umask to xxx for directories42 ============ ========================================43 44Disk format45===========46 47OMFS discriminates between "sysblocks" and normal data blocks. The sysblock48group consists of super block information, file metadata, directory structures,49and extents. Each sysblock has a header containing CRCs of the entire50sysblock, and may be mirrored in successive blocks on the disk. A sysblock may51have a smaller size than a data block, but since they are both addressed by the52same 64-bit block number, any remaining space in the smaller sysblock is53unused.54 55Sysblock header information::56 57 struct omfs_header {58 __be64 h_self; /* FS block where this is located */59 __be32 h_body_size; /* size of useful data after header */60 __be16 h_crc; /* crc-ccitt of body_size bytes */61 char h_fill1[2];62 u8 h_version; /* version, always 1 */63 char h_type; /* OMFS_INODE_X */64 u8 h_magic; /* OMFS_IMAGIC */65 u8 h_check_xor; /* XOR of header bytes before this */66 __be32 h_fill2;67 };68 69Files and directories are both represented by omfs_inode::70 71 struct omfs_inode {72 struct omfs_header i_head; /* header */73 __be64 i_parent; /* parent containing this inode */74 __be64 i_sibling; /* next inode in hash bucket */75 __be64 i_ctime; /* ctime, in milliseconds */76 char i_fill1[35];77 char i_type; /* OMFS_[DIR,FILE] */78 __be32 i_fill2;79 char i_fill3[64];80 char i_name[OMFS_NAMELEN]; /* filename */81 __be64 i_size; /* size of file, in bytes */82 };83 84Directories in OMFS are implemented as a large hash table. Filenames are85hashed then prepended into the bucket list beginning at OMFS_DIR_START.86Lookup requires hashing the filename, then seeking across i_sibling pointers87until a match is found on i_name. Empty buckets are represented by block88pointers with all-1s (~0).89 90A file is an omfs_inode structure followed by an extent table beginning at91OMFS_EXTENT_START::92 93 struct omfs_extent_entry {94 __be64 e_cluster; /* start location of a set of blocks */95 __be64 e_blocks; /* number of blocks after e_cluster */96 };97 98 struct omfs_extent {99 __be64 e_next; /* next extent table location */100 __be32 e_extent_count; /* total # extents in this table */101 __be32 e_fill;102 struct omfs_extent_entry e_entry; /* start of extent entries */103 };104 105Each extent holds the block offset followed by number of blocks allocated to106the extent. The final extent in each table is a terminator with e_cluster107being ~0 and e_blocks being ones'-complement of the total number of blocks108in the table.109 110If this table overflows, a continuation inode is written and pointed to by111e_next. These have a header but lack the rest of the inode structure.112 113