2.6 KiB
libx52 Hotplug Support
This document describes a design for USB hotplug support in libx52. The idea is to have a daemon process running at all times, which will update the hardware when the X52/X52Pro is plugged in.
Assumptions
The core assumption is that a maximum of 1 device is present at any time. There is no goal to support more than 1 X52/X52Pro at the same time.
LibUSB Hotplug Support
libusb-1.0 version 1.0.16 and newer support hotplug notifications when a device is inserted or removed. These call a registered callback function which can read the device descriptor, and take action accordingly.
libx52 Hotplug design
The hotplug mechanism consists of the following functions:
int libx52_hotplug_init(libx52_hotplug_service **svc);
void libx52_hotplug_exit(libx52_hotplug_service *svc);
int libx52_hotplug_register_callback(libx52_hotplug_service *svc,
libx52_hotplug_fn *callback_fn,
void *user_data,
libx52_hotplug_callback_handle **cb_handle);
int libx52_hotplug_deregister_callback(libx52_hotplug_callback_handle *cb_handle);
typedef void (*libx52_hotplug_fn)(bool inserted, void *user_data, libx52_device *dev);
The init function will take care of initializing libx52, so the user should
not call libx52_init. The exit function will also take care of deinitializing
libx52, so the user should not call libx52_exit.
The user may register any number of callbacks, and they are called in an undefined order on device insertion and removal. Each register callback also returns a callback handle in an output parameter. This callback handle can be used to deregister the callback in the future, if necessary.
libx52_hotplug_exit will take care of deregistering all registered callbacks,
and will deinitialize libx52.
Interaction with libusbx52
As of this writing, libusbx52 reads from a regular file, however, in order to support hotplug, the read should read from a fifo. Under Linux, a program can open a fifo for reading and writing, and this should allow the program to write a small amount of data into the fifo and exit, and the program using libusbx52 can read from that file.
In this case, the data format should change to include an additional character prior to the idVendor and idProduct fields. This additional character will indicate whether the device is being inserted or removed.
libusbx52 must now spawn a separate thread to deal with the fifo. This thread will read from the fifo, and callback the registered handler when an event occurs.