* Overview

  Mass Storage Gadget (or MSG) acts as a USB Mass Storage device,
  appearing to the host as a disk or a CD-ROM drive.  It supports
  multiple logical units (LUNs).  Backing storage for each LUN is
  provided by a regular file or a block device, access can be limited
  to read-only, and gadget can indicate that it is removable and/or
  CD-ROM (the latter implies read-only access).

  Its requirements are modest; only a bulk-in and a bulk-out endpoint
  are needed.  The memory requirement amounts to two 16K buffers.
  Support is included for full-speed, high-speed and SuperSpeed
  operation.

  Note that the driver is slightly non-portable in that it assumes
  a single memory/DMA buffer will be usable for bulk-in and bulk-out
  endpoints.  With most device controllers this is not an issue, but
  there may be some with hardware restrictions that prevent a buffer
  from being used by more than one endpoint.

  This document describes how to use the gadget from user space, its
  relation to mass storage function (or MSF) and different gadgets
  using it, and how it differs from File Storage Gadget (or FSG)
  (which is no longer included in Linux).  It will talk only briefly
  about how to use MSF within composite gadgets.

* Module parameters

  The mass storage gadget accepts the following mass storage specific
  module parameters:

  - file=filename[,filename...]

    This parameter lists paths to files or block devices used for
    backing storage for each logical unit.  There may be at most
    FSG_MAX_LUNS (8) LUNs set.  If more files are specified, they will
    be silently ignored.  See also “luns” parameter.

    *BEWARE* that if a file is used as a backing storage, it may not
    be modified by any other process.  This is because the host
    assumes the data does not change without its knowledge.  It may be
    read, but (if the logical unit is writable) due to buffering on
    the host side, the contents are not well defined.

    The size of the logical unit will be rounded down to a full
    logical block.  The logical block size is 2048 bytes for LUNs
    simulating CD-ROM, block size of the device if the backing file is
    a block device, or 512 bytes otherwise.

  - removable=b[,b...]

    This parameter specifies whether each logical unit should be
    removable.  “b” here is either “y”, “Y” or “1” for true or “n”,
    “N” or “0” for false.

    If this option is set for a logical unit, gadget will accept an
    “eject” SCSI request (Start/Stop Unit).  When it is sent, the
    backing file will be closed to simulate ejection and the logical
    unit will not be mountable by the host until a new backing file is
    specified by userspace on the device (see “sysfs entries”
    section).

    If a logical unit is not removable (the default), a backing file
    must be specified for it with the “file” parameter as the module
    is loaded.  The same applies if the module is built in, no
    exceptions.

    The default value of the flag is false, *HOWEVER* it used to be
    true.  This has been changed to better match File Storage Gadget
    and because it seems like a saner default after all.  Thus to
    maintain compatibility with older kernels, it's best to specify
    the default values.  Also, if one relied on old default, explicit
    “n” needs to be specified now.

    Note that “removable” means the logical unit's media can be
    ejected or removed (as is true for a CD-ROM drive or a card
    reader).  It does *not* mean that the entire gadget can be
    unplugged from the host; the proper term for that is
    “hot-unpluggable”.

  - cdrom=b[,b...]

    This parameter specifies whether each logical unit should simulate
    CD-ROM.  The default is false.

  - ro=b[,b...]

    This parameter specifies whether each logical unit should be
    reported as read only.  This will prevent host from modifying the
    backing files.

    Note that if this flag for given logical unit is false but the
    backing file could not be opened in read/write mode, the gadget
    will fall back to read only mode anyway.

    The default value for non-CD-ROM logical units is false; for
    logical units simulating CD-ROM it is forced to true.

  - nofua=b[,b...]

    This parameter specifies whether FUA flag should be ignored in SCSI
    Write10 and Write12 commands sent to given logical units.

    MS Windows mounts removable storage in “Removal optimised mode” by
    default.  All the writes to the media are synchronous, which is
    achieved by setting the FUA (Force Unit Access) bit in SCSI
    Write(10,12) commands.  This forces each write to wait until the
    data has actually been written out and prevents I/O requests
    aggregation in block layer dramatically decreasing performance.

    Note that this may mean that if the device is powered from USB and
    the user unplugs the device without unmounting it first (which at
    least some Windows users do), the data may be lost.

    The default value is false.

  - luns=N

    This parameter specifies number of logical units the gadget will
    have.  It is limited by FSG_MAX_LUNS (8) and higher value will be
    capped.

    If this parameter is provided, and the number of files specified
    in “file” argument is greater then the value of “luns”, all excess
    files will be ignored.

    If this parameter is not present, the number of logical units will
    be deduced from the number of files specified in the “file”
    parameter.  If the file parameter is missing as well, one is
    assumed.

  - stall=b

    Specifies whether the gadget is allowed to halt bulk endpoints.
    The default is determined according to the type of USB device
    controller, but usually true.

  In addition to the above, the gadget also accepts the following
  parameters defined by the composite framework (they are common to
  all composite gadgets so just a quick listing):

  - idVendor      -- USB Vendor ID (16 bit integer)
  - idProduct     -- USB Product ID (16 bit integer)
  - bcdDevice     -- USB Device version (BCD) (16 bit integer)
  - iManufacturer -- USB Manufacturer string (string)
  - iProduct      -- USB Product string (string)
  - iSerialNumber -- SerialNumber string (sting)

* sysfs entries

  For each logical unit, the gadget creates a directory in the sysfs
  hierarchy.  Inside of it the following three files are created:

  - file

    When read it returns the path to the backing file for the given
    logical unit.  If there is no backing file (possible only if the
    logical unit is removable), the content is empty.

    When written into, it changes the backing file for given logical
    unit.  This change can be performed even if given logical unit is
    not specified as removable (but that may look strange to the
    host).  It may fail, however, if host disallowed medium removal
    with the Prevent-Allow Medium Removal SCSI command.

  - ro

    Reflects the state of ro flag for the given logical unit.  It can
    be read any time, and written to when there is no backing file
    open for given logical unit.

  - nofua

    Reflects the state of nofua flag for given logical unit.  It can
    be read and written.

  Other then those, as usual, the values of module parameters can be
  read from /sys/module/g_mass_storage/parameters/* files.

* Other gadgets using mass storage function

  The Mass Storage Gadget uses the Mass Storage Function to handle
  mass storage protocol.  As a composite function, MSF may be used by
  other gadgets as well (eg. g_multi and acm_ms).

  All of the information in previous sections are valid for other
  gadgets using MSF, except that support for mass storage related
  module parameters may be missing, or the parameters may have
  a prefix.  To figure out whether any of this is true one needs to
  consult the gadget's documentation or its source code.

  For examples of how to include mass storage function in gadgets, one
  may take a look at mass_storage.c, acm_ms.c and multi.c (sorted by
  complexity).

* Relation to file storage gadget

  The Mass Storage Function and thus the Mass Storage Gadget has been
  based on the File Storage Gadget.  The difference between the two is
  that MSG is a composite gadget (ie. uses the composite framework)
  while file storage gadget was a traditional gadget.  From userspace
  point of view this distinction does not really matter, but from
  kernel hacker's point of view, this means that (i) MSG does not
  duplicate code needed for handling basic USB protocol commands and
  (ii) MSF can be used in any other composite gadget.

  Because of that, File Storage Gadget has been removed in Linux 3.8.
  All users need to transition to the Mass Storage Gadget.  The two
  gadgets behave mostly the same from the outside except:

  1. In FSG the “removable” and “cdrom” module parameters set the flag
     for all logical units whereas in MSG they accept a list of y/n
     values for each logical unit.  If one uses only a single logical
     unit this does not matter, but if there are more, the y/n value
     needs to be repeated for each logical unit.

  2. FSG's “serial”, “vendor”, “product” and “release” module
     parameters are handled in MSG by the composite layer's parameters
     named respectively: “iSerialnumber”, “idVendor”, “idProduct” and
     “bcdDevice”.

  3. MSG does not support FSG's test mode, thus “transport”,
     “protocol” and “buflen” FSG's module parameters are not
     supported.  MSG always uses SCSI protocol with bulk only
     transport mode and 16 KiB buffers.