--- /dev/null
+* 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 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, and how it differs 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 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 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.
* function for a USB device, it also illustrates a technique of
* double-buffering for increased throughput.
*
- * Function supports multiple logical units (LUNs). Backing storage
- * for each LUN is provided by a regular file or a block device.
- * Access for each LUN can be limited to read-only. Moreover, the
- * function can indicate that LUN is removable and/or CD-ROM. (The
- * later implies read-only access.)
- *
+ * For more information about MSF and in particular its module
+ * parameters and sysfs interface read the
+ * <Documentation/usb/mass-storage.txt> file.
+ */
+
+/*
* MSF is configured by specifying a fsg_config structure. It has the
* following fields:
*
* data track and no audio tracks; hence there need be only one
* backing file per LUN.
*
- *
- * MSF includes support for module parameters. If gadget using it
- * decides to use it, the following module parameters will be
- * available:
- *
- * file=filename[,filename...]
- * Names of the files or block devices used for
- * backing storage.
- * ro=b[,b...] Default false, boolean for read-only access.
- * removable=b[,b...]
- * Default false, boolean for removable media.
- * cdrom=b[,b...] Default false, boolean for whether to emulate
- * a CD-ROM drive.
- * nofua=b[,b...] Default false, booleans for ignore FUA flag
- * in SCSI WRITE(10,12) commands
- * luns=N Default N = number of filenames, number of
- * LUNs to support.
- * stall Default determined according to the type of
- * USB device controller (usually true),
- * boolean to permit the driver to halt
- * bulk endpoints.
- *
- * The module parameters may be prefixed with some string. You need
- * to consult gadget's documentation or source to verify whether it is
- * using those module parameters and if it does what are the prefixes
- * (look for FSG_MODULE_PARAMETERS() macro usage, what's inside it is
- * the prefix).
- *
- *
- * Requirements are modest; only a bulk-in and a bulk-out endpoint are
- * needed. The memory requirement amounts to two 16K buffers, size
- * configurable by a parameter. Support is included for both
- * full-speed and high-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, bulk-out, and
- * interrupt-in endpoints. With most device controllers this isn't an
- * issue, but there may be some with hardware restrictions that prevent
- * a buffer from being used by more than one endpoint.
- *
- *
- * The pathnames of the backing files, the ro settings and nofua
- * settings are available in the attribute files "file", "ro" and
- * "nofua" in the lun<n> subdirectory of the gadget's sysfs directory.
- * If the "removable" option is set, writing to these files will
- * simulate ejecting/loading the medium (writing an empty line means
- * eject) and adjusting a write-enable tab. Changes to the ro setting
- * are not allowed when the medium is loaded or if CD-ROM emulation is
- * being used.
- *
- * When a LUN receive an "eject" SCSI request (Start/Stop Unit),
- * if the LUN is removable, the backing file is released to simulate
- * ejection.
- *
- *
* This function is heavily based on "File-backed Storage Gadget" by
* Alan Stern which in turn is heavily based on "Gadget Zero" by David
* Brownell. The driver's SCSI command interface was based on the
* In normal operation the main thread is started during the gadget's
* fsg_bind() callback and stopped during fsg_unbind(). But it can
* also exit when it receives a signal, and there's no point leaving
- * the gadget running when the thread is dead. At of this moment, MSF
+ * the gadget running when the thread is dead. As of this moment, MSF
* provides no way to deregister the gadget when thread dies -- maybe
* a callback functions is needed.
*
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