Input Ring

The memory address of the input ring is registered with an ioctl (or fp_ioctl) subroutine call. The program that registers the input ring is the owner of the ring and is responsible for allocating, initializing, and freeing the storage associated with the ring. The same input ring can be shared by multiple devices.

The input ring consists of the input ring header followed by the reporting area. The input ring header contains the reporting area size, the head pointer, the tail pointer, the overflow flag, and the notification type flag. Before registering an input ring, the ring owner must ensure that the head and tail pointers contain the starting address of the reporting area. The overflow flag must also be cleared and the size field set equal to the number of bytes in the reporting area. After the input ring has been registered, the owner can modify only the head pointer and the notification type flag.

Data stored on the input ring is structured as one or more event reports. Event reports are placed at the tail of the ring by the graphic input device drivers. Previously queued event reports are taken from the head of the input ring by the owner of the ring. The input ring is empty when the head and tail locations are the same. An overflow condition exists if placement of an event on the input ring would overwrite data that has not been processed. Following an overflow, new event reports are not placed on the input ring until the input ring is flushed via an ioctl subroutine or service vector call.

The owner of the input ring is notified when an event is available for processing via a SIGMSG signal or via callback if the channel was created by an fp_open subroutine call. The notification type flag in the input ring header specifies whether the owner should be notified each tine an event is placed on the ring or only when an event is placed on an empty ring.