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Diffstat (limited to 'libevdev/libevdev.h')
| -rw-r--r-- | libevdev/libevdev.h | 45 |
1 files changed, 45 insertions, 0 deletions
diff --git a/libevdev/libevdev.h b/libevdev/libevdev.h index 9ec1cb8..ef142ea 100644 --- a/libevdev/libevdev.h +++ b/libevdev/libevdev.h @@ -368,6 +368,51 @@ extern "C" { * The axis events do not reflect the position of a current touch point, a * client must take care not to generate a new touch point based on those * updates. + * + * Discarding events before synchronizing + * ===================================== + * + * The kernel implements the client buffer as a ring buffer. SYN_DROPPED + * events are handled when the buffer is full and a new event is received + * from a device. All existing events are discarded, a SYN_DROPPED is added + * to the buffer followed by the actual device event. Further events will be + * appended to the buffer until it is either read by the client, or filled + * again, at which point the sequence repeats. + * + * When the client reads the buffer, the buffer will thus always consist of + * exactly one SYN_DROPPED event followed by an unspecified number of real + * events. The data the ioctls return is the current state of the device, + * i.e. the state after all these events have been processed. For example, + * assume the buffer contains the following sequence: + * + * @code + EV_SYN SYN_DROPPED + EV_ABS ABS_X 1 + EV_SYN SYN_REPORT 0 + EV_ABS ABS_X 2 + EV_SYN SYN_REPORT 0 + EV_ABS ABS_X 3 + EV_SYN SYN_REPORT 0 + EV_ABS ABS_X 4 + EV_SYN SYN_REPORT 0 + EV_ABS ABS_X 5 + EV_SYN SYN_REPORT 0 + EV_ABS ABS_X 6 + EV_SYN SYN_REPORT 0 + * @endcode + * An ioctl at any time in this sequence will return a value of 6 for ABS_X. + * + * libevdev discards all events after a SYN_DROPPED to ensure the events + * during @ref LIBEVDEV_READ_FLAG_SYNC represent the last known state of the + * device. This loses some granularity of the events especially as the time + * between the SYN_DROPPED and the sync process increases. It does however + * avoid spurious cursor movements. In the above example, the event sequence + * by libevdev is: + * @code + EV_SYN SYN_DROPPED + EV_ABS ABS_X 6 + EV_SYN SYN_REPORT 0 + @endcode */ /** |