xref: /linux/Documentation/usb/mass-storage.rst (revision 17cfcb68af3bc7d5e8ae08779b1853310a2949f3)
1=========================
2Mass Storage Gadget (MSG)
3=========================
4
5Overview
6========
7
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).
14
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.
19
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.
25
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.
31
32Module parameters
33=================
34
35  The mass storage gadget accepts the following mass storage specific
36  module parameters:
37
38  - file=filename[,filename...]
39
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.
44
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.
50
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.
55
56  - removable=b[,b...]
57
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.
61
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).
68
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.
73
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.
80
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”.
86
87  - cdrom=b[,b...]
88
89    This parameter specifies whether each logical unit should simulate
90    CD-ROM.  The default is false.
91
92  - ro=b[,b...]
93
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.
97
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.
101
102    The default value for non-CD-ROM logical units is false; for
103    logical units simulating CD-ROM it is forced to true.
104
105  - nofua=b[,b...]
106
107    This parameter specifies whether FUA flag should be ignored in SCSI
108    Write10 and Write12 commands sent to given logical units.
109
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.
116
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.
120
121    The default value is false.
122
123  - luns=N
124
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.
128
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.
132
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.
137
138  - stall=b
139
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.
143
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):
147
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)
154
155sysfs entries
156=============
157
158  For each logical unit, the gadget creates a directory in the sysfs
159  hierarchy.  Inside of it the following three files are created:
160
161  - file
162
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.
166
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.
172
173  - ro
174
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.
178
179  - nofua
180
181    Reflects the state of nofua flag for given logical unit.  It can
182    be read and written.
183
184  Other then those, as usual, the values of module parameters can be
185  read from /sys/module/g_mass_storage/parameters/* files.
186
187Other gadgets using mass storage function
188=========================================
189
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).
193
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.
199
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).
203
204Relation to file storage gadget
205===============================
206
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.
215
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:
219
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.
225
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”.
230
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.
235