This is the main documentation for the CVF-FAT filesystem extension.  31DEC1997


Table of Contents:

1. The idea of CVF-FAT
2. Restrictions
3. Mount options
4. Description of the CVF-FAT interface
5. CVF Modules

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1. The idea of CVF-FAT
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CVF-FAT is a FAT filesystem extension that provides a generic interface for
Compressed Volume Files in FAT partitions. Popular CVF software, for
example, are Microsoft's Doublespace/Drivespace and Stac's Stacker.
Using the CVF-FAT interface, it is possible to load a module that handles
all the low-level disk access that has to do with on-the-fly compression
and decompression. Any other part of FAT filesystem access is still handled
by the FAT, MSDOS or VFAT or even UMSDOS driver.

CVF access works by redirecting certain low-level routines from the FAT
driver to a loadable, CVF-format specific module. This module must fake
a normal FAT filesystem to the FAT driver while doing all the extra stuff
like compression and decompression silently.


2. Restrictions
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- BMAP problems

  CVF filesystems cannot do bmap. It's impossible in principle. Thus
  all actions that require bmap do not work (swapping, writable mmapping).
  Read-only mmapping works because the FAT driver has a hack for this
  situation :) Well, with some tricks writable mmapping could work,
  (proof: they did under old dmsdos), but..... (hint: readpage/writepage
  interface functions) ...... but the FAT driver has to support them 
  first without bmap :-)

  We'll see. If someone points me to an application that needs this, I
  might be persuaded to implement it :). CVF-FAT is already prepared
  for using readpage.
  
- attention, DOSEmu users 

  You may have to unmount all CVF partitions before running DOSEmu depending 
  on your configuration. If DOSEmu is configured to use wholedisk or 
  partition access (this is often the case to let DOSEmu access 
  compressed partitions) there's a risk of destroying your compressed 
  partitions or crashing your system because of confused drivers.
  
  Note that it is always safe to redirect the compressed partitions with 
  lredir or emufs.sys. Refer to the DOSEmu documentation for details.


3. Mount options
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The CVF-FAT extension currently adds the following options to the FAT
driver's standard options:

  cvf_format=xxx
    Forces the driver to use the CVF module "xxx" instead of auto-detection.
    This is only necessary if the CVF format is not recognized correctly
    because of bugs or incompatibilities in the CVF modules. (It skips
    the detect_cvf call.) "xxx" may be the text "none" (without the quotes)
    to inhibit using any of the loaded CVF modules, just in case a CVF
    module insists on mounting plain FAT filesystems by misunderstanding :)

  cvf_options=yyy
    Option string passed to the CVF module. I.e. only the "yyy" is passed
    (without the quotes). The documentation for each CVF module should 
    explain it since it is interpreted only by the CVF module. Note that 
    the string must not contain a comma (",") - this would lead to 
    misinterpretation by the FAT driver, which would recognize the text 
    after a comma as a FAT driver option and might get confused or print 
    strange error messages. The documentation for the CVF module should 
    offer a different separation symbol, for example the dot ".", which
    is only valid inside the string "yyy".


4. Description of the CVF-FAT interface
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Assuming you want to write your own CVF module, you need to write a lot of
interface functions. Most of them are covered in the kernel documentation
you can find on the net, and thus won't be described here. They have been
marked with "[...]" :-) Take a look at include/linux/fat_cvf.h.

struct cvf_format
{ int cvf_version;
  char* cvf_version_text;
  unsigned long int flags;
  int (*detect_cvf) (struct super_block*sb);
  int (*mount_cvf) (struct super_block*sb,char*options);
  int (*unmount_cvf) (struct super_block*sb);
  [...]
  void (*cvf_zero_cluster) (struct inode*inode,int clusternr);
}

This structure defines the capabilities of a CVF module. It must be filled
out completely by a CVF module. Consider it as a kind of form that is used
to introduce the module to the FAT/CVF-FAT driver.

It contains...
  - cvf_version:
      A version id which must be unique. Choose one.
  - cvf_version_text:
      A human readable version string that should be one short word 
      describing the CVF format the module implements. This text is used
      for the cvf_format option. This name must also be unique.
  - flags:
      Bit coded flags, currently only used for a readpage/mmap hack that 
      provides both mmap and readpage functionality. If CVF_USE_READPAGE
      is set, mmap is set to generic_file_mmap and readpage is caught
      and redirected to the cvf_readpage function. If it is not set,
      readpage is set to generic_readpage and mmap is caught and redirected
      to cvf_mmap.
  - detect_cvf:
      A function that is called to decide whether the filesystem is a CVF of
      the type the module supports. The detect_cvf function must return 0
      for "NO, I DON'T KNOW THIS GARBAGE" or anything !=0 for "YES, THIS IS
      THE KIND OF CVF I SUPPORT". The function must maintain the module
      usage counters for safety, i.e. do MOD_INC_USE_COUNT at the beginning
      and MOD_DEC_USE_COUNT at the end. The function *must not* assume that
      successful recongition would lead to a call of the mount_cvf function
      later. 
  - mount_cvf:
      A function that sets up some values or initializes something additional
      to what has to be done when a CVF is mounted. This is called at the
      end of fat_read_super and must return 0 on success. Definitely, this
      function must increment the module usage counter by MOD_INC_USE_COUNT.
      This mount_cvf function is also responsible for interpreting a CVF
      module specific option string (the "yyy" from the FAT mount option
      "cvf_options=yyy") which cannot contain a comma (use for example the
      dot "." as option separator symbol).
  - unmount_cvf:
      A function that is called when the filesystem is unmounted. Most likely
      it only frees up some memory and calls MOD_DEC_USE_COUNT. The return
      value might be ignored (it currently is ignored).
  - [...]:
      All other interface functions are "caught" FAT driver functions, i.e.
      are executed by the FAT driver *instead* of the original FAT driver
      functions. NULL means use the original FAT driver functions instead.
      If you really want "no action", write a function that does nothing and 
      hang it in instead.
  - cvf_zero_cluster:
      The cvf_zero_cluster function is called when the fat driver wants to
      zero out a (new) cluster. This is important for directories (mkdir).
      If it is NULL, the FAT driver defaults to overwriting the whole
      cluster with zeros. Note that clusternr is absolute, not relative
      to the provided inode.

Notes:
  1. The cvf_bmap function should be ignored. It really should never
     get called from somewhere. I recommend redirecting it to a panic
     or fatal error message so bugs show up immediately.
  2. The cvf_writepage function is ignored. This is because the fat
     driver doesn't support it. This might change in future. I recommend
     setting it to NULL (i.e use default).

int register_cvf_format(struct cvf_format*cvf_format);
  If you have just set up a variable containing the above structure,
  call this function to introduce your CVF format to the FAT/CVF-FAT
  driver. This is usually done in init_module. Be sure to check the
  return value. Zero means success, everything else causes a kernel
  message printed in the syslog describing the error that occurred.
  Typical errors are:
    - a module with the same version id is already registered or 
    - too many CVF formats. Hack fs/fat/cvf.c if you need more.

int unregister_cvf_format(struct cvf_format*cvf_format);
  This is usually called in cleanup_module. Return value =0 means
  success. An error only occurs if you try to unregister a CVF format
  that has not been previously registered. The code uses the version id
  to distinguish the modules, so be sure to keep it unique.

5. CVS Modules
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Refer to the dmsdos module (the successor of the dmsdos filesystem) for a
sample implementation.  It can currently be found at

  ftp://fb9nt.uni-duisburg.de/pub/linux/dmsdos

