+// For any card that's not the master (where we pick out the frames as they
+// come, as fast as we can process), there's going to be a queue. The question
+// is when we should drop frames from that queue (apart from the obvious
+// dropping if the 16-frame queue should become full), especially given that
+// the frame rate could be lower or higher than the master (either subtly or
+// dramatically). We have two (conflicting) demands:
+//
+// 1. We want to avoid starving the queue.
+// 2. We don't want to add more delay than is needed.
+//
+// Our general strategy is to drop as many frames as we can (helping for #2)
+// that we think is safe for #1 given jitter. To this end, we set a lower floor N,
+// where we assume that if we have N frames in the queue, we're always safe from
+// starvation. (Typically, N will be 0 or 1. It starts off at 0.) If we have
+// more than N frames in the queue after reading out the one we need, we head-drop
+// them to reduce the queue.
+//
+// N is reduced as follows: If the queue has had at least one spare frame for
+// at least 50 (master) frames (ie., it's been too conservative for a second),
+// we reduce N by 1 and reset the timers. TODO: Only do this if N ever actually
+// touched the limit.
+//
+// Whenever the queue is starved (we needed a frame but there was none),
+// and we've been at N since the last starvation, N was obviously too low,
+// so we increment it. We will never set N above 5, though.
+class QueueLengthPolicy {
+public:
+ QueueLengthPolicy() {}
+ void reset(unsigned card_index) {
+ this->card_index = card_index;
+ safe_queue_length = 0;
+ frames_with_at_least_one = 0;
+ been_at_safe_point_since_last_starvation = false;
+ }
+
+ void update_policy(int queue_length); // Give in -1 for starvation.
+ unsigned get_safe_queue_length() const { return safe_queue_length; }
+
+private:
+ unsigned card_index; // For debugging only.
+ unsigned safe_queue_length = 0; // Called N in the comments.
+ unsigned frames_with_at_least_one = 0;
+ bool been_at_safe_point_since_last_starvation = false;
+};
+