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+
+
+
+
+		Linux USB gadget configured through configfs
+
+
+			     25th April 2013
+
+
+
+
+Overview
+========
+
+A USB Linux Gadget is a device which has a UDC (USB Device Controller) and can
+be connected to a USB Host to extend it with additional functions like a serial
+port or a mass storage capability.
+
+A gadget is seen by its host as a set of configurations, each of which contains
+a number of interfaces which, from the gadget's perspective, are known as
+functions, each function representing e.g. a serial connection or a SCSI disk.
+
+Linux provides a number of functions for gadgets to use.
+
+Creating a gadget means deciding what configurations there will be
+and which functions each configuration will provide.
+
+Configfs (please see Documentation/filesystems/configfs/*) lends itself nicely
+for the purpose of telling the kernel about the above mentioned decision.
+This document is about how to do it.
+
+It also describes how configfs integration into gadget is designed.
+
+
+
+
+Requirements
+============
+
+In order for this to work configfs must be available, so CONFIGFS_FS must be
+'y' or 'm' in .config. As of this writing USB_LIBCOMPOSITE selects CONFIGFS_FS.
+
+
+
+
+Usage
+=====
+
+(The original post describing the first function
+made available through configfs can be seen here:
+http://www.spinics.net/lists/linux-usb/msg76388.html)
+
+$ modprobe libcomposite
+$ mount none $CONFIGFS_HOME -t configfs
+
+where CONFIGFS_HOME is the mount point for configfs
+
+1. Creating the gadgets
+-----------------------
+
+For each gadget to be created its corresponding directory must be created:
+
+$ mkdir $CONFIGFS_HOME/usb_gadget/<gadget name>
+
+e.g.:
+
+$ mkdir $CONFIGFS_HOME/usb_gadget/g1
+
+...
+...
+...
+
+$ cd $CONFIGFS_HOME/usb_gadget/g1
+
+Each gadget needs to have its vendor id <VID> and product id <PID> specified:
+
+$ echo <VID> > idVendor
+$ echo <PID> > idProduct
+
+A gadget also needs its serial number, manufacturer and product strings.
+In order to have a place to store them, a strings subdirectory must be created
+for each language, e.g.:
+
+$ mkdir strings/0x409
+
+Then the strings can be specified:
+
+$ echo <serial number> > strings/0x409/serialnumber
+$ echo <manufacturer> > strings/0x409/manufacturer
+$ echo <product> > strings/0x409/product
+
+2. Creating the configurations
+------------------------------
+
+Each gadget will consist of a number of configurations, their corresponding
+directories must be created:
+
+$ mkdir configs/<name>.<number>
+
+where <name> can be any string which is legal in a filesystem and the
+<number> is the configuration's number, e.g.:
+
+$ mkdir configs/c.1
+
+...
+...
+...
+
+Each configuration also needs its strings, so a subdirectory must be created
+for each language, e.g.:
+
+$ mkdir configs/c.1/strings/0x409
+
+Then the configuration string can be specified:
+
+$ echo <configuration> > configs/c.1/strings/0x409/configuration
+
+Some attributes can also be set for a configuration, e.g.:
+
+$ echo 120 > configs/c.1/MaxPower
+
+3. Creating the functions
+-------------------------
+
+The gadget will provide some functions, for each function its corresponding
+directory must be created:
+
+$ mkdir functions/<name>.<instance name>
+
+where <name> corresponds to one of allowed function names and instance name
+is an arbitrary string allowed in a filesystem, e.g.:
+
+$ mkdir functions/ncm.usb0 # usb_f_ncm.ko gets loaded with request_module()
+
+...
+...
+...
+
+Each function provides its specific set of attributes, with either read-only
+or read-write access. Where applicable they need to be written to as
+appropriate.
+Please refer to Documentation/ABI/*/configfs-usb-gadget* for more information.
+
+4. Associating the functions with their configurations
+------------------------------------------------------
+
+At this moment a number of gadgets is created, each of which has a number of
+configurations specified and a number of functions available. What remains
+is specifying which function is available in which configuration (the same
+function can be used in multiple configurations). This is achieved with
+creating symbolic links:
+
+$ ln -s functions/<name>.<instance name> configs/<name>.<number>
+
+e.g.:
+
+$ ln -s functions/ncm.usb0 configs/c.1
+
+...
+...
+...
+
+5. Enabling the gadget
+----------------------
+
+All the above steps serve the purpose of composing the gadget of
+configurations and functions.
+
+An example directory structure might look like this:
+
+.
+./strings
+./strings/0x409
+./strings/0x409/serialnumber
+./strings/0x409/product
+./strings/0x409/manufacturer
+./configs
+./configs/c.1
+./configs/c.1/ncm.usb0 -> ../../../../usb_gadget/g1/functions/ncm.usb0
+./configs/c.1/strings
+./configs/c.1/strings/0x409
+./configs/c.1/strings/0x409/configuration
+./configs/c.1/bmAttributes
+./configs/c.1/MaxPower
+./functions
+./functions/ncm.usb0
+./functions/ncm.usb0/ifname
+./functions/ncm.usb0/qmult
+./functions/ncm.usb0/host_addr
+./functions/ncm.usb0/dev_addr
+./UDC
+./bcdUSB
+./bcdDevice
+./idProduct
+./idVendor
+./bMaxPacketSize0
+./bDeviceProtocol
+./bDeviceSubClass
+./bDeviceClass
+
+
+Such a gadget must be finally enabled so that the USB host can enumerate it.
+In order to enable the gadget it must be bound to a UDC (USB Device Controller).
+
+$ echo <udc name> > UDC
+
+where <udc name> is one of those found in /sys/class/udc/*
+e.g.:
+
+$ echo s3c-hsotg > UDC
+
+
+6. Disabling the gadget
+-----------------------
+
+$ echo "" > UDC
+
+7. Cleaning up
+--------------
+
+Remove functions from configurations:
+
+$ rm configs/<config name>.<number>/<function>
+
+where <config name>.<number> specify the configuration and <function> is
+a symlink to a function being removed from the configuration, e.g.:
+
+$ rm configfs/c.1/ncm.usb0
+
+...
+...
+...
+
+Remove strings directories in configurations
+
+$ rmdir configs/<config name>.<number>/strings/<lang>
+
+e.g.:
+
+$ rmdir configs/c.1/strings/0x409
+
+...
+...
+...
+
+and remove the configurations
+
+$ rmdir configs/<config name>.<number>
+
+e.g.:
+
+rmdir configs/c.1
+
+...
+...
+...
+
+Remove functions (function modules are not unloaded, though)
+
+$ rmdir functions/<name>.<instance name>
+
+e.g.:
+
+$ rmdir functions/ncm.usb0
+
+...
+...
+...
+
+Remove strings directories in the gadget
+
+$ rmdir strings/<lang>
+
+e.g.:
+
+$ rmdir strings/0x409
+
+and finally remove the gadget:
+
+$ cd ..
+$ rmdir <gadget name>
+
+e.g.:
+
+$ rmdir g1
+
+
+
+
+Implementation design
+=====================
+
+Below the idea of how configfs works is presented.
+In configfs there are items and groups, both represented as directories.
+The difference between an item and a group is that a group can contain
+other groups. In the picture below only an item is shown.
+Both items and groups can have attributes, which are represented as files.
+The user can create and remove directories, but cannot remove files,
+which can be read-only or read-write, depending on what they represent.
+
+The filesystem part of configfs operates on config_items/groups and
+configfs_attributes which are generic and of the same type for all
+configured elements. However, they are embedded in usage-specific
+larger structures. In the picture below there is a "cs" which contains
+a config_item and an "sa" which contains a configfs_attribute.
+
+The filesystem view would be like this:
+
+./
+./cs        (directory)
+   |
+   +--sa    (file)
+   |
+   .
+   .
+   .
+
+Whenever a user reads/writes the "sa" file, a function is called
+which accepts a struct config_item and a struct configfs_attribute.
+In the said function the "cs" and "sa" are retrieved using the well
+known container_of technique and an appropriate sa's function (show or
+store) is called and passed the "cs" and a character buffer. The "show"
+is for displaying the file's contents (copy data from the cs to the
+buffer), while the "store" is for modifying the file's contents (copy data
+from the buffer to the cs), but it is up to the implementer of the
+two functions to decide what they actually do.
+
+typedef struct configured_structure cs;
+typedef struct specific_attribute sa;
+
+                                       sa
+                       +----------------------------------+
+        cs             |  (*show)(cs *, buffer);          |
++-----------------+    |  (*store)(cs *, buffer, length); |
+|                 |    |                                  |
+| +-------------+ |    |       +------------------+       |
+| | struct      |-|----|------>|struct            |       |
+| | config_item | |    |       |configfs_attribute|       |
+| +-------------+ |    |       +------------------+       |
+|                 |    +----------------------------------+
+| data to be set  |                .
+|                 |                .
++-----------------+                .
+
+The file names are decided by the config item/group designer, while
+the directories in general can be named at will. A group can have
+a number of its default sub-groups created automatically.
+
+For more information on configfs please see
+Documentation/filesystems/configfs/*.
+
+The concepts described above translate to USB gadgets like this:
+
+1. A gadget has its config group, which has some attributes (idVendor,
+idProduct etc) and default sub-groups (configs, functions, strings).
+Writing to the attributes causes the information to be stored in
+appropriate locations. In the configs, functions and strings sub-groups
+a user can create their sub-groups to represent configurations, functions,
+and groups of strings in a given language.
+
+2. The user creates configurations and functions, in the configurations
+creates symbolic links to functions. This information is used when the
+gadget's UDC attribute is written to, which means binding the gadget
+to the UDC. The code in drivers/usb/gadget/configfs.c iterates over
+all configurations, and in each configuration it iterates over all
+functions and binds them. This way the whole gadget is bound.
+
+3. The file drivers/usb/gadget/configfs.c contains code for
+
+	- gadget's config_group
+	- gadget's default groups (configs, functions, strings)
+	- associating functions with configurations (symlinks)
+
+4. Each USB function naturally has its own view of what it wants
+configured, so config_groups for particular functions are defined
+in the functions implementation files drivers/usb/gadget/f_*.c.
+
+5. Function's code is written in such a way that it uses
+
+usb_get_function_instance(), which, in turn, calls request_module.
+So, provided that modprobe works, modules for particular functions
+are loaded automatically. Please note that the converse is not true:
+after a gadget is disabled and torn down, the modules remain loaded.