235 lines
9.7 KiB
ReStructuredText
235 lines
9.7 KiB
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=========================
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Mass Storage Gadget (MSG)
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=========================
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Overview
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========
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Mass Storage Gadget (or MSG) acts as a USB Mass Storage device,
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appearing to the host as a disk or a CD-ROM drive. It supports
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multiple logical units (LUNs). Backing storage for each LUN is
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provided by a regular file or a block device, access can be limited
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to read-only, and gadget can indicate that it is removable and/or
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CD-ROM (the latter implies read-only access).
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Its requirements are modest; only a bulk-in and a bulk-out endpoint
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are needed. The memory requirement amounts to two 16K buffers.
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Support is included for full-speed, high-speed and SuperSpeed
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operation.
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Note that the driver is slightly non-portable in that it assumes
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a single memory/DMA buffer will be usable for bulk-in and bulk-out
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endpoints. With most device controllers this is not an issue, but
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there may be some with hardware restrictions that prevent a buffer
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from being used by more than one endpoint.
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This document describes how to use the gadget from user space, its
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relation to mass storage function (or MSF) and different gadgets
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using it, and how it differs from File Storage Gadget (or FSG)
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(which is no longer included in Linux). It will talk only briefly
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about how to use MSF within composite gadgets.
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Module parameters
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=================
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The mass storage gadget accepts the following mass storage specific
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module parameters:
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- file=filename[,filename...]
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This parameter lists paths to files or block devices used for
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backing storage for each logical unit. There may be at most
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FSG_MAX_LUNS (8) LUNs set. If more files are specified, they will
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be silently ignored. See also “luns” parameter.
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*BEWARE* that if a file is used as a backing storage, it may not
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be modified by any other process. This is because the host
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assumes the data does not change without its knowledge. It may be
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read, but (if the logical unit is writable) due to buffering on
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the host side, the contents are not well defined.
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The size of the logical unit will be rounded down to a full
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logical block. The logical block size is 2048 bytes for LUNs
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simulating CD-ROM, block size of the device if the backing file is
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a block device, or 512 bytes otherwise.
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- removable=b[,b...]
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This parameter specifies whether each logical unit should be
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removable. “b” here is either “y”, “Y” or “1” for true or “n”,
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“N” or “0” for false.
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If this option is set for a logical unit, gadget will accept an
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“eject” SCSI request (Start/Stop Unit). When it is sent, the
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backing file will be closed to simulate ejection and the logical
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unit will not be mountable by the host until a new backing file is
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specified by userspace on the device (see “sysfs entries”
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section).
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If a logical unit is not removable (the default), a backing file
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must be specified for it with the “file” parameter as the module
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is loaded. The same applies if the module is built in, no
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exceptions.
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The default value of the flag is false, *HOWEVER* it used to be
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true. This has been changed to better match File Storage Gadget
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and because it seems like a saner default after all. Thus to
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maintain compatibility with older kernels, it's best to specify
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the default values. Also, if one relied on old default, explicit
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“n” needs to be specified now.
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Note that “removable” means the logical unit's media can be
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ejected or removed (as is true for a CD-ROM drive or a card
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reader). It does *not* mean that the entire gadget can be
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unplugged from the host; the proper term for that is
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“hot-unpluggable”.
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- cdrom=b[,b...]
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This parameter specifies whether each logical unit should simulate
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CD-ROM. The default is false.
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- ro=b[,b...]
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This parameter specifies whether each logical unit should be
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reported as read only. This will prevent host from modifying the
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backing files.
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Note that if this flag for given logical unit is false but the
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backing file could not be opened in read/write mode, the gadget
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will fall back to read only mode anyway.
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The default value for non-CD-ROM logical units is false; for
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logical units simulating CD-ROM it is forced to true.
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- nofua=b[,b...]
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This parameter specifies whether FUA flag should be ignored in SCSI
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Write10 and Write12 commands sent to given logical units.
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MS Windows mounts removable storage in “Removal optimised mode” by
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default. All the writes to the media are synchronous, which is
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achieved by setting the FUA (Force Unit Access) bit in SCSI
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Write(10,12) commands. This forces each write to wait until the
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data has actually been written out and prevents I/O requests
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aggregation in block layer dramatically decreasing performance.
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Note that this may mean that if the device is powered from USB and
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the user unplugs the device without unmounting it first (which at
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least some Windows users do), the data may be lost.
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The default value is false.
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- luns=N
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This parameter specifies number of logical units the gadget will
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have. It is limited by FSG_MAX_LUNS (8) and higher value will be
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capped.
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If this parameter is provided, and the number of files specified
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in “file” argument is greater then the value of “luns”, all excess
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files will be ignored.
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If this parameter is not present, the number of logical units will
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be deduced from the number of files specified in the “file”
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parameter. If the file parameter is missing as well, one is
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assumed.
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- stall=b
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Specifies whether the gadget is allowed to halt bulk endpoints.
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The default is determined according to the type of USB device
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controller, but usually true.
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In addition to the above, the gadget also accepts the following
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parameters defined by the composite framework (they are common to
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all composite gadgets so just a quick listing):
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- idVendor -- USB Vendor ID (16 bit integer)
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- idProduct -- USB Product ID (16 bit integer)
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- bcdDevice -- USB Device version (BCD) (16 bit integer)
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- iManufacturer -- USB Manufacturer string (string)
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- iProduct -- USB Product string (string)
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- iSerialNumber -- SerialNumber string (sting)
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sysfs entries
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=============
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For each logical unit, the gadget creates a directory in the sysfs
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hierarchy. Inside of it the following three files are created:
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- file
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When read it returns the path to the backing file for the given
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logical unit. If there is no backing file (possible only if the
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logical unit is removable), the content is empty.
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When written into, it changes the backing file for given logical
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unit. This change can be performed even if given logical unit is
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not specified as removable (but that may look strange to the
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host). It may fail, however, if host disallowed medium removal
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with the Prevent-Allow Medium Removal SCSI command.
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- ro
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Reflects the state of ro flag for the given logical unit. It can
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be read any time, and written to when there is no backing file
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open for given logical unit.
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- nofua
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Reflects the state of nofua flag for given logical unit. It can
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be read and written.
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Other then those, as usual, the values of module parameters can be
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read from /sys/module/g_mass_storage/parameters/* files.
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Other gadgets using mass storage function
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=========================================
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The Mass Storage Gadget uses the Mass Storage Function to handle
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mass storage protocol. As a composite function, MSF may be used by
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other gadgets as well (eg. g_multi and acm_ms).
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All of the information in previous sections are valid for other
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gadgets using MSF, except that support for mass storage related
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module parameters may be missing, or the parameters may have
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a prefix. To figure out whether any of this is true one needs to
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consult the gadget's documentation or its source code.
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For examples of how to include mass storage function in gadgets, one
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may take a look at mass_storage.c, acm_ms.c and multi.c (sorted by
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complexity).
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Relation to file storage gadget
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===============================
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The Mass Storage Function and thus the Mass Storage Gadget has been
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based on the File Storage Gadget. The difference between the two is
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that MSG is a composite gadget (ie. uses the composite framework)
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while file storage gadget was a traditional gadget. From userspace
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point of view this distinction does not really matter, but from
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kernel hacker's point of view, this means that (i) MSG does not
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duplicate code needed for handling basic USB protocol commands and
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(ii) MSF can be used in any other composite gadget.
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Because of that, File Storage Gadget has been removed in Linux 3.8.
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All users need to transition to the Mass Storage Gadget. The two
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gadgets behave mostly the same from the outside except:
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1. In FSG the “removable” and “cdrom” module parameters set the flag
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for all logical units whereas in MSG they accept a list of y/n
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values for each logical unit. If one uses only a single logical
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unit this does not matter, but if there are more, the y/n value
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needs to be repeated for each logical unit.
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2. FSG's “serial”, “vendor”, “product” and “release” module
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parameters are handled in MSG by the composite layer's parameters
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named respectively: “iSerialnumber”, “idVendor”, “idProduct” and
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“bcdDevice”.
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3. MSG does not support FSG's test mode, thus “transport”,
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“protocol” and “buflen” FSG's module parameters are not
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supported. MSG always uses SCSI protocol with bulk only
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transport mode and 16 KiB buffers.
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