fs: initial qnx6fs addition

The QNX6 Filesystem

===================

The qnx6fs is used by newer QNX operating system versions. (e.g. Neutrino)

It got introduced in QNX 6.4.0 and is used default since 6.4.1.

Option

======

mmi_fs		Mount filesystem as used for example by Audi MMI 3G system

Specification

=============

qnx6fs shares many properties with traditional Unix filesystems. It has the

concepts of blocks, inodes and directories.

On QNX it is possible to create little endian and big endian qnx6 filesystems.

This feature makes it possible to create and use a different endianness fs

for the target (QNX is used on quite a range of embedded systems) plattform

running on a different endianess.

The Linux driver handles endianness transparently. (LE and BE)

Blocks

------

The space in the device or file is split up into blocks. These are a fixed

size of 512, 1024, 2048 or 4096, which is decided when the filesystem is

created.

Blockpointers are 32bit, so the maximum space that can be adressed is

2^32 * 4096 bytes or 16TB

The superblocks

---------------

The superblock contains all global information about the filesystem.

Each qnx6fs got two superblocks, each one having a 64bit serial number.

That serial number is used to identify the "active" superblock.

In write mode with reach new snapshot (after each synchronous write), the

serial of the new master superblock is increased (old superblock serial + 1)

So basically the snapshot functionality is realized by an atomic final

update of the serial number. Before updating that serial, all modifications

are done by copying all modified blocks during that specific write request

(or period) and building up a new (stable) filesystem structure under the

inactive superblock.

Each superblock holds a set of root inodes for the different filesystem

parts. (Inode, Bitmap and Longfilenames)

Each of these root nodes holds information like total size of the stored

data and the adressing levels in that specific tree.

If the level value is 0, up to 16 direct blocks can be adressed by each

node.

Level 1 adds an additional indirect adressing level where each indirect

adressing block holds up to blocksize / 4 bytes pointers to data blocks.

Level 2 adds an additional indirect adressig block level (so, already up

to 16 * 256 * 256 = 1048576 blocks that can be adressed by such a tree)a

Unused block pointers are always set to ~0 - regardless of root node,

indirect adressing blocks or inodes.

Data leaves are always on the lowest level. So no data is stored on upper

tree levels.

The first Superblock is located at 0x2000. (0x2000 is the bootblock size)

The Audi MMI 3G first superblock directly starts at byte 0.

Second superblock position can either be calculated from the superblock

information (total number of filesystem blocks) or by taking the highest

device address, zeroing the last 3 bytes and then substracting 0x1000 from

that address.

0x1000 is the size reserved for each superblock - regardless of the

blocksize of the filesystem.

Inodes

------

Each object in the filesystem is represented by an inode. (index node)

The inode structure contains pointers to the filesystem blocks which contain

the data held in the object and all of the metadata about an object except

its longname. (filenames longer than 27 characters)

The metadata about an object includes the permissions, owner, group, flags,

size, number of blocks used, access time, change time and modification time.

Object mode field is POSIX format. (which makes things easier)

There are also pointers to the first 16 blocks, if the object data can be

adressed with 16 direct blocks.

For more than 16 blocks an indirect adressing in form of another tree is

used. (scheme is the same as the one used for the superblock root nodes)

The filesize is stored 64bit. Inode counting starts with 1. (whilst long

filename inodes start with 0)

Directories

-----------

A directory is a filesystem object and has an inode just like a file.

It is a specially formatted file containing records which associate each

name with an inode number.

'.' inode number points to the directory inode

'..' inode number points to the parent directory inode

Eeach filename record additionally got a filename length field.

One special case are long filenames or subdirectory names.

These got set a filename length field of 0xff in the corresponding directory

record plus the longfile inode number also stored in that record.

With that longfilename inode number, the longfilename tree can be walked

starting with the superblock longfilename root node pointers.

Special files

-------------

Symbolic links are also filesystem objects with inodes. They got a specific

bit in the inode mode field identifying them as symbolic link.

The directory entry file inode pointer points to the target file inode.

Hard links got an inode, a directory entry, but a specific mode bit set,

no block pointers and the directory file record pointing to the target file

inode.

Character and block special devices do not exist in QNX as those files

are handled by the QNX kernel/drivers and created in /dev independant of the

underlaying filesystem.

Long filenames

--------------

Long filenames are stored in a seperate adressing tree. The staring point

is the longfilename root node in the active superblock.

Each data block (tree leaves) holds one long filename. That filename is

limited to 510 bytes. The first two starting bytes are used as length field

for the actual filename.

If that structure shall fit for all allowed blocksizes, it is clear why there

is a limit of 510 bytes for the actual filename stored.

Bitmap

------

The qnx6fs filesystem allocation bitmap is stored in a tree under bitmap

root node in the superblock and each bit in the bitmap represents one

filesystem block.

The first block is block 0, which starts 0x1000 after superblock start.

So for a normal qnx6fs 0x3000 (bootblock + superblock) is the physical

address at which block 0 is located.

Bits at the end of the last bitmap block are set to 1, if the device is

smaller than addressing space in the bitmap.

Bitmap system area

------------------

The bitmap itself is devided into three parts.

First the system area, that is split into two halfs.

Then userspace.

The requirement for a static, fixed preallocated system area comes from how

qnx6fs deals with writes.

Each superblock got it's own half of the system area. So superblock #1

always uses blocks from the lower half whilst superblock #2 just writes to

blocks represented by the upper half bitmap system area bits.

Bitmap blocks, Inode blocks and indirect addressing blocks for those two

tree structures are treated as system blocks.

The rational behind that is that a write request can work on a new snapshot

(system area of the inactive - resp. lower serial numbered superblock) while

at the same time there is still a complete stable filesystem structer in the

other half of the system area.

When finished with writing (a sync write is completed, the maximum sync leap

time or a filesystem sync is requested), serial of the previously inactive

superblock atomically is increased and the fs switches over to that - then

stable declared - superblock.

For all data outside the system area, blocks are just copied while writing.

source "fs/omfs/Kconfig"

source "fs/hpfs/Kconfig"

source "fs/qnx4/Kconfig"

source "fs/qnx6/Kconfig"

source "fs/romfs/Kconfig"

source "fs/pstore/Kconfig"

source "fs/sysv/Kconfig"

obj-$(CONFIG_AFFS_FS)		+= affs/

obj-$(CONFIG_ROMFS_FS)		+= romfs/

obj-$(CONFIG_QNX4FS_FS)		+= qnx4/

obj-$(CONFIG_QNX6FS_FS)		+= qnx6/

obj-$(CONFIG_AUTOFS4_FS)	+= autofs4/

obj-$(CONFIG_ADFS_FS)		+= adfs/

obj-$(CONFIG_FUSE_FS)		+= fuse/

config QNX6FS_FS

	tristate "QNX6 file system support (read only)"

	depends on BLOCK && CRC32

	help

	  This is the file system used by the real-time operating systems

	  QNX 6 (also called QNX RTP).

	  Further information is available at <http://www.qnx.com/>.

	  Say Y if you intend to mount QNX hard disks or floppies formatted

          with a mkqnx6fs.

	  However, keep in mind that this currently is a readonly driver!

	  To compile this file system support as a module, choose M here: the

	  module will be called qnx6.

	  If you don't know whether you need it, then you don't need it:

	  answer N.

config QNX6FS_DEBUG

	bool "QNX6 debugging information"

	depends on QNX6FS_FS

	help

	  Turns on extended debugging output.

	  If you are not a developer working on the QNX6FS, you probably don't

	  want this:

	  answer N.

#

# Makefile for the linux qnx4-filesystem routines.

#

obj-$(CONFIG_QNX6FS_FS) += qnx6.o

qnx6-objs := inode.o dir.o namei.o super_mmi.o