A credit-based shaping algorithm is available on the two highest priority queues and is defined in the standard 802.1Qav: Forwarding and Queuing Enhancements for Time-Sensitive Streams. This allows traffic on these queues to be limited and to allow other queues to transmit.
Traffic shaping is enabled via the CBS (Credit Based Shaping) Control register. This enables a counter which stores the amount of transmit 'credit', measured in bytes that a particular queue has. A queue may only transmit if it has non-negative credit. If a queue has data to send, but is held off from doing as another queue is transmitting, then credit will accumulate in the credit counter at the rate defined in the IdleSlope register (GMAC_CBSISQx) for that queue.
portTransmitRate is the transmission rate, in bits per second, that the underlying MAC service that supports transmission through the Port provides. The value of this parameter is determined by the operation of the MAC. IdleSlope is the rate of change of increasing credit when waiting to transmit and must be less than the value of the portTransmitRate.
IdleSlope is the rate of change of credit when waiting to transmit and must be less than the value of the portTransmitRate.
The max value of IdleSlope (or sendSlope) is (portTransmitRate / bits_per_MII_Clock).
In case of 100 Mbps, maximum IdleSlope = (100 Mbps / 4) = 0x17D7840.
When this queue is transmitting the credit counter is decremented at the rate of sendSlope which is defined as (portTransmitRate - IdleSlope). A queue can accumulate negative credit when transmitting which will hold off any other transfers from that queue until credit returns to a non-negative value. No transfers are halted when a queue's credit becomes negative; it will accumulate negative credit until the transfer completes.
The highest priority queue always has priority regardless of which queue has the most credit.