How does QoS with priority and ordering allowed with AXI ID?

I have one question for QoS with AXI4:

Can one master have multiple QoS values?

Meaning one master issues requests with different QoS values but all with same AXI ID and the interconnect network arbitrates between these requests based on QoS value and may give response out of order. I see an issue that if the AXI IDs are same for request and the responses are out of order then it may violate the in order requirement within an ID.

Possible solution will be

a. One master is issuing unique QoS value

b. If one master is issuing multiple QoS values then the AXI IDs will be different for  those request.

Let me know if there are better solutions for QoS in AXI4 with AXI IDs.

Best Regards,

Tarun

  • Hi Tarun,

    It's common for a master to change its QoS value if it needs different levels of service from the system. For example if there is a bandwidth regulator that changes the QoS value based on a target bandwidth.

    AXI ordering rules will take precedence and must be met for AXI compatibility. QoS is used for arbitration decisions, but protocol rules are most important. Many modern AXI masters will use multiple different AXI IDs to get more performance by allowing the memory system to post more outstanding requests, which could then be returned out of order.

    Neil.

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