2Mass Storage Gadget (MSG)
   8  Mass Storage Gadget (or MSG) acts as a USB Mass Storage device,
   9  appearing to the host as a disk or a CD-ROM drive.  It supports
  10  multiple logical units (LUNs).  Backing storage for each LUN is
  11  provided by a regular file or a block device, access can be limited
  12  to read-only, and gadget can indicate that it is removable and/or
  13  CD-ROM (the latter implies read-only access).
  15  Its requirements are modest; only a bulk-in and a bulk-out endpoint
  16  are needed.  The memory requirement amounts to two 16K buffers.
  17  Support is included for full-speed, high-speed and SuperSpeed
  18  operation.
  20  Note that the driver is slightly non-portable in that it assumes
  21  a single memory/DMA buffer will be usable for bulk-in and bulk-out
  22  endpoints.  With most device controllers this is not an issue, but
  23  there may be some with hardware restrictions that prevent a buffer
  24  from being used by more than one endpoint.
  26  This document describes how to use the gadget from user space, its
  27  relation to mass storage function (or MSF) and different gadgets
  28  using it, and how it differs from File Storage Gadget (or FSG)
  29  (which is no longer included in Linux).  It will talk only briefly
  30  about how to use MSF within composite gadgets.
  32Module parameters
  35  The mass storage gadget accepts the following mass storage specific
  36  module parameters:
  38  - file=filename[,filename...]
  40    This parameter lists paths to files or block devices used for
  41    backing storage for each logical unit.  There may be at most
  42    FSG_MAX_LUNS (8) LUNs set.  If more files are specified, they will
  43    be silently ignored.  See also “luns” parameter.
  45    *BEWARE* that if a file is used as a backing storage, it may not
  46    be modified by any other process.  This is because the host
  47    assumes the data does not change without its knowledge.  It may be
  48    read, but (if the logical unit is writable) due to buffering on
  49    the host side, the contents are not well defined.
  51    The size of the logical unit will be rounded down to a full
  52    logical block.  The logical block size is 2048 bytes for LUNs
  53    simulating CD-ROM, block size of the device if the backing file is
  54    a block device, or 512 bytes otherwise.
  56  - removable=b[,b...]
  58    This parameter specifies whether each logical unit should be
  59    removable.  “b” here is either “y”, “Y” or “1” for true or “n”,
  60    “N” or “0” for false.
  62    If this option is set for a logical unit, gadget will accept an
  63    “eject” SCSI request (Start/Stop Unit).  When it is sent, the
  64    backing file will be closed to simulate ejection and the logical
  65    unit will not be mountable by the host until a new backing file is
  66    specified by userspace on the device (see “sysfs entries”
  67    section).
  69    If a logical unit is not removable (the default), a backing file
  70    must be specified for it with the “file” parameter as the module
  71    is loaded.  The same applies if the module is built in, no
  72    exceptions.
  74    The default value of the flag is false, *HOWEVER* it used to be
  75    true.  This has been changed to better match File Storage Gadget
  76    and because it seems like a saner default after all.  Thus to
  77    maintain compatibility with older kernels, it's best to specify
  78    the default values.  Also, if one relied on old default, explicit
  79    “n” needs to be specified now.
  81    Note that “removable” means the logical unit's media can be
  82    ejected or removed (as is true for a CD-ROM drive or a card
  83    reader).  It does *not* mean that the entire gadget can be
  84    unplugged from the host; the proper term for that is
  85    “hot-unpluggable”.
  87  - cdrom=b[,b...]
  89    This parameter specifies whether each logical unit should simulate
  90    CD-ROM.  The default is false.
  92  - ro=b[,b...]
  94    This parameter specifies whether each logical unit should be
  95    reported as read only.  This will prevent host from modifying the
  96    backing files.
  98    Note that if this flag for given logical unit is false but the
  99    backing file could not be opened in read/write mode, the gadget
 100    will fall back to read only mode anyway.
 102    The default value for non-CD-ROM logical units is false; for
 103    logical units simulating CD-ROM it is forced to true.
 105  - nofua=b[,b...]
 107    This parameter specifies whether FUA flag should be ignored in SCSI
 108    Write10 and Write12 commands sent to given logical units.
 110    MS Windows mounts removable storage in “Removal optimised mode” by
 111    default.  All the writes to the media are synchronous, which is
 112    achieved by setting the FUA (Force Unit Access) bit in SCSI
 113    Write(10,12) commands.  This forces each write to wait until the
 114    data has actually been written out and prevents I/O requests
 115    aggregation in block layer dramatically decreasing performance.
 117    Note that this may mean that if the device is powered from USB and
 118    the user unplugs the device without unmounting it first (which at
 119    least some Windows users do), the data may be lost.
 121    The default value is false.
 123  - luns=N
 125    This parameter specifies number of logical units the gadget will
 126    have.  It is limited by FSG_MAX_LUNS (8) and higher value will be
 127    capped.
 129    If this parameter is provided, and the number of files specified
 130    in “file” argument is greater then the value of “luns”, all excess
 131    files will be ignored.
 133    If this parameter is not present, the number of logical units will
 134    be deduced from the number of files specified in the “file”
 135    parameter.  If the file parameter is missing as well, one is
 136    assumed.
 138  - stall=b
 140    Specifies whether the gadget is allowed to halt bulk endpoints.
 141    The default is determined according to the type of USB device
 142    controller, but usually true.
 144  In addition to the above, the gadget also accepts the following
 145  parameters defined by the composite framework (they are common to
 146  all composite gadgets so just a quick listing):
 148  - idVendor      -- USB Vendor ID (16 bit integer)
 149  - idProduct     -- USB Product ID (16 bit integer)
 150  - bcdDevice     -- USB Device version (BCD) (16 bit integer)
 151  - iManufacturer -- USB Manufacturer string (string)
 152  - iProduct      -- USB Product string (string)
 153  - iSerialNumber -- SerialNumber string (sting)
 155sysfs entries
 158  For each logical unit, the gadget creates a directory in the sysfs
 159  hierarchy.  Inside of it the following three files are created:
 161  - file
 163    When read it returns the path to the backing file for the given
 164    logical unit.  If there is no backing file (possible only if the
 165    logical unit is removable), the content is empty.
 167    When written into, it changes the backing file for given logical
 168    unit.  This change can be performed even if given logical unit is
 169    not specified as removable (but that may look strange to the
 170    host).  It may fail, however, if host disallowed medium removal
 171    with the Prevent-Allow Medium Removal SCSI command.
 173  - ro
 175    Reflects the state of ro flag for the given logical unit.  It can
 176    be read any time, and written to when there is no backing file
 177    open for given logical unit.
 179  - nofua
 181    Reflects the state of nofua flag for given logical unit.  It can
 182    be read and written.
 184  Other then those, as usual, the values of module parameters can be
 185  read from /sys/module/g_mass_storage/parameters/* files.
 187Other gadgets using mass storage function
 190  The Mass Storage Gadget uses the Mass Storage Function to handle
 191  mass storage protocol.  As a composite function, MSF may be used by
 192  other gadgets as well (eg. g_multi and acm_ms).
 194  All of the information in previous sections are valid for other
 195  gadgets using MSF, except that support for mass storage related
 196  module parameters may be missing, or the parameters may have
 197  a prefix.  To figure out whether any of this is true one needs to
 198  consult the gadget's documentation or its source code.
 200  For examples of how to include mass storage function in gadgets, one
 201  may take a look at mass_storage.c, acm_ms.c and multi.c (sorted by
 202  complexity).
 204Relation to file storage gadget
 207  The Mass Storage Function and thus the Mass Storage Gadget has been
 208  based on the File Storage Gadget.  The difference between the two is
 209  that MSG is a composite gadget (ie. uses the composite framework)
 210  while file storage gadget was a traditional gadget.  From userspace
 211  point of view this distinction does not really matter, but from
 212  kernel hacker's point of view, this means that (i) MSG does not
 213  duplicate code needed for handling basic USB protocol commands and
 214  (ii) MSF can be used in any other composite gadget.
 216  Because of that, File Storage Gadget has been removed in Linux 3.8.
 217  All users need to transition to the Mass Storage Gadget.  The two
 218  gadgets behave mostly the same from the outside except:
 220  1. In FSG the “removable” and “cdrom” module parameters set the flag
 221     for all logical units whereas in MSG they accept a list of y/n
 222     values for each logical unit.  If one uses only a single logical
 223     unit this does not matter, but if there are more, the y/n value
 224     needs to be repeated for each logical unit.
 226  2. FSG's “serial”, “vendor”, “product” and “release” module
 227     parameters are handled in MSG by the composite layer's parameters
 228     named respectively: “iSerialnumber”, “idVendor”, “idProduct” and
 229     “bcdDevice”.
 231  3. MSG does not support FSG's test mode, thus “transport”,
 232     “protocol” and “buflen” FSG's module parameters are not
 233     supported.  MSG always uses SCSI protocol with bulk only
 234     transport mode and 16 KiB buffers.