The primary function of Smart Remote is to reliably collect the information about user input and transmit it to the host. The main difficulty arises from the fact that due to a limited power budget, the device spends most of its time in low power mode, in which it is not connected to the host. When the user presses a button, the device must wake up, connect to the host, and transmit the information about the button press up the link.
HID state subsystem is responsible for tracking the device connection state and properly routing input events, either to the BLE communication subsystem or to the event queue.
HID events can be divided into two main categories:
Depending on the category, HID state subsystem treats events differently. The matrix below presents this behavior.
| Input Event Category \ BLE Connection State | Disconnected | Connected |
|---|---|---|
| Absolute | enqueue | if (queue_is_not_empty) enqueue else trasmit |
| Relative | drop | transmit |
When the device is disconnected and an incoming input event cannot be dropped, the event is passed to the HID event queue. The number of the elements in the queue is limited by CONFIG_PROTOCOL_HID_EVENT_QUEUE_SIZE. The events are automatically discarded after CONFIG_HID_REPORT_EXPIRATION milliseconds. When this happens, the following warning message is logged (where %u is substituted with the number of events that were discarded).
It is important that the order in which events are enqueued is maintained. For this reason, an expired event cannot be discarded if any of the events prior to it is not discarded as well. To maintain state sanity, a button down event can only be discarded when a matching button up event is discarded as well. If there is no place in the queue to store an incoming event, the HID state subsystem tries to discard the oldest event in the queue.
The above rules are matched in a general case when keys are pressed and released in a sequence one after another. In this case, the oldest key press and release is discarded, and a new event is stored. When no events are generated and no connection is established, events are discarded after the expiration time defined above. However, if the user presses and holds the button, while simultaneously generating a sequence of other button events, the queue cannot discard events and an overflow happens. To maintain state sanity, all events are dropped in such case. The following warning message is logged.
When the device is connected, all incoming events are scheduled for transmission. This is done through an update of an entry in the event item list and notification sent to the BLE communication subsystem. When the BLE communication subsystem is ready for HID events transmission, it obtains the information about elements present on the event item list, and for each element it constructs a proper entry in the HID report. Elements on the list are present as long as there is any information that causes the report entry to be generated (for example, a button is pressed or mouse position coordinates are updated). When the value associated with the event drops to zero (which means, the button is released or there is no mouse movement), the item is discarded from the list.
The number of items that are recorded by HID state subsystem is configured with CONFIG_PROTOCOL_HID_STATE_ITEM_COUNT. In case input modules generate more active events at the same time (for example, the keyboard module issues information about more keys being pressed than there are available elements in the list), the incoming event is discarded and the following message is logged.