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Diffstat (limited to 'Documentation/usb')
| -rw-r--r-- | Documentation/usb/index.rst | 1 | ||||
| -rw-r--r-- | Documentation/usb/raw-gadget.rst | 61 |
2 files changed, 62 insertions, 0 deletions
diff --git a/Documentation/usb/index.rst b/Documentation/usb/index.rst index 36b6ebd9a9d9..b656c9be23ed 100644 --- a/Documentation/usb/index.rst +++ b/Documentation/usb/index.rst @@ -22,6 +22,7 @@ USB support misc_usbsevseg mtouchusb ohci + raw-gadget usbip_protocol usbmon usb-serial diff --git a/Documentation/usb/raw-gadget.rst b/Documentation/usb/raw-gadget.rst new file mode 100644 index 000000000000..9e78cb858f86 --- /dev/null +++ b/Documentation/usb/raw-gadget.rst @@ -0,0 +1,61 @@ +============== +USB Raw Gadget +============== + +USB Raw Gadget is a kernel module that provides a userspace interface for +the USB Gadget subsystem. Essentially it allows to emulate USB devices +from userspace. Enabled with CONFIG_USB_RAW_GADGET. Raw Gadget is +currently a strictly debugging feature and shouldn't be used in +production, use GadgetFS instead. + +Comparison to GadgetFS +~~~~~~~~~~~~~~~~~~~~~~ + +Raw Gadget is similar to GadgetFS, but provides a more low-level and +direct access to the USB Gadget layer for the userspace. The key +differences are: + +1. Every USB request is passed to the userspace to get a response, while + GadgetFS responds to some USB requests internally based on the provided + descriptors. However note, that the UDC driver might respond to some + requests on its own and never forward them to the Gadget layer. + +2. GadgetFS performs some sanity checks on the provided USB descriptors, + while Raw Gadget allows you to provide arbitrary data as responses to + USB requests. + +3. Raw Gadget provides a way to select a UDC device/driver to bind to, + while GadgetFS currently binds to the first available UDC. + +4. Raw Gadget uses predictable endpoint names (handles) across different + UDCs (as long as UDCs have enough endpoints of each required transfer + type). + +5. Raw Gadget has ioctl-based interface instead of a filesystem-based one. + +Userspace interface +~~~~~~~~~~~~~~~~~~~ + +To create a Raw Gadget instance open /dev/raw-gadget. Multiple raw-gadget +instances (bound to different UDCs) can be used at the same time. The +interaction with the opened file happens through the ioctl() calls, see +comments in include/uapi/linux/usb/raw_gadget.h for details. + +The typical usage of Raw Gadget looks like: + +1. Open Raw Gadget instance via /dev/raw-gadget. +2. Initialize the instance via USB_RAW_IOCTL_INIT. +3. Launch the instance with USB_RAW_IOCTL_RUN. +4. In a loop issue USB_RAW_IOCTL_EVENT_FETCH calls to receive events from + Raw Gadget and react to those depending on what kind of USB device + needs to be emulated. + +Potential future improvements +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +- Implement ioctl's for setting/clearing halt status on endpoints. + +- Reporting more events (suspend, resume, etc.) through + USB_RAW_IOCTL_EVENT_FETCH. + +- Support O_NONBLOCK I/O. |