AM335x ICSS EMAC LLD driver collision queue

Hi RY,

Your understanding is correct. In Switch Mode, collision/queue contention happens when both the PRUs or a PRU and Host want to acquire the same queue at same time. It can happen as follows:

Queue Contention on Host Queue:
Queue contention happens on a host queue when both the PRUs try to acquire the same queue (for example, queue 3) to store a frame at the same time. Only one PRU can hold the queue at one point of time. Lets say, if PRU0 takes hold of queue 3 first, PRU1 will then store in the collision queue and vice versa.

Queue Contention on Port Queue:
Queue contention happens on a port queue when Host and PRU try to acquire a queue to store the frame at the same time. This can happen on the queues of both the ports:
• On Port1, contention can happen between Host and PRU1.
• On Port2, contention can happen between Host and PRU0.

Queue Contention Resolution
When queue contention happens then one of the core will store/queue the frame in the collision queue. Frame stored in the collision queue has to be moved to the main queue. Host always receives the frames through the four 'host receive queues' and frames are always transmitted from the four 'port transmit queues' on both the ports. So, if there is a frame waiting in collision queue of Port1 then it is moved to one of the transmit queue of Port1 by the PRU1. If there is a frame waiting in collicion queue of Port2 then it is moved to one of the transmit queue of Port 2 by PRU0.

Collision queue can have maximum one frame at a time.

Regards,
Anjandeep