diff options
Diffstat (limited to 'Documentation/usb/gadget_configfs.rst')
| -rw-r--r-- | Documentation/usb/gadget_configfs.rst | 390 |
1 files changed, 390 insertions, 0 deletions
diff --git a/Documentation/usb/gadget_configfs.rst b/Documentation/usb/gadget_configfs.rst new file mode 100644 index 000000000..158e48dab --- /dev/null +++ b/Documentation/usb/gadget_configfs.rst @@ -0,0 +1,390 @@ +============================================ +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.rst`) 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 configs/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.rst`. + +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. |