allocated by the xHCI host controller driver. Each ring is comprised of one or more segments, which consists of 16-byte Transfer Request Blocks (TRBs) that can be chained to form a Transfer Descriptor (TD) that represents a multiple-buffer request. Segments are linked into a ring using Link TRBs, which means they are dynamically growable. The producer of the ring enqueues a TD by writing one or more TRBs in the ring and toggling the TRB cycle bit for each TRB. The consumer knows it can process the TRB when the cycle bit matches its internal consumer cycle state for the ring. The consumer cycle state is toggled an odd amount of times in the ring. An example ring (a ring must have a minimum of 16 TRBs on it, but that's too big to draw in ASCII art): chain cycle bit bit ------------------------ | TD A TRB 1 | 1 | 1 |<------------- <-- consumer dequeue ptr ------------------------ | consumer cycle state = 1 | TD A TRB 2 | 1 | 1 | | ------------------------ | | TD A TRB 3 | 0 | 1 | segment 1 | ------------------------ | | TD B TRB 1 | 1 | 1 | | ------------------------ | | TD B TRB 2 | 0 | 1 | | ------------------------ | | Link TRB | 0 | 1 |----- | ------------------------ | | | | chain cycle | | bit bit | | ------------------------ | | | TD C TRB 1 | 0 | 1 |<---- | ------------------------ | | TD D TRB 1 | 1 | 1 | | ------------------------ | | TD D TRB 2 | 1 | 1 | segment 2 | ------------------------ | | TD D TRB 3 | 1 | 1 | | ------------------------ | | TD D TRB 4 | 1 | 1 | | ------------------------ | | Link TRB | 1 | 1 |----- | ------------------------ | | | | chain cycle | | bit bit | | ------------------------ | | | TD D TRB 5 | 1 | 1 |<---- | ------------------------ | | TD D TRB 6 | 0 | 1 | | ------------------------ | | TD E TRB 1 | 0 | 1 | segment 3 | ------------------------ | | | 0 | 0 | | <-- producer enqueue ptr ------------------------ | | | 0 | 0 | | ------------------------ | | Link TRB | 0 | 0 |--------------- ------------------------ Signed-off-by: Sarah Sharp Signed-off-by: Greg Kroah-Hartman ç