Minor g_mass_storage improvements.

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+                                                                   -*- org -*-
+
+* 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 super-speed
+  operation.
+
+  Note that the driver is slightly non-portable in that it assumes
+  a single memory/DMA buffer will be useable 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, as well as and how does it differ from File Storage Gadget
+  (or FSG).  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 host assumes
+    the data does not change without its knowledge.  It may by 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 full logical
+    block.  A logical block size is 2048 for LUNs simulating CD-ROM,
+    block size of a device if the device is the backing file, 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 released 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 logical unit is not removable (the default), backing file must
+    be specified for it with the “file” 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.
+
+  - 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 each logical unit is false.
+
+  - 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 FUA (Force Unit Access) bit in SCSI
+    Write(10,12) commands.  This 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
+    user unplugs the device without unmounting (which at lest 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 deducted 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 given
+    logical unit.  If there is no backing file (possible only if
+    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 Allow Medium Removal SCSI command.
+
+  - ro
+
+    Reflects the state of ro flag for 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 to change it.
+
+  Other then those, as usual, the values of modules 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 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 is 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 deprecated and
+  scheduled to be removed in Linux 3.8.  All users need to transition
+  to the Mass Storage Gadget by that time.  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.