I'm trying to understand about QoS of AXI bus operation.
I thought that QoS will be processed priority and re-ordering with values of QoS as the below.
awqos 1..8..d..a..3..2..6..2
arqos 1..2..2..3..6..8..a..d
But awqos and arqos both seems like to work as an "in-order" in the capture below.
https://imgur.com/a/C1qNlOc
arqos 1..8..d..a..3..2..6..2
Does anyone help me understand about QoS operation and why it operates " in-order" instead of "re-order"?
If I want to see the QoS operation, what am I supposed to do?
From an AXI Specification point of view, AxQoS is used to indicate the priority of requests where a higher value is a higher priority.
Exactly how this is used depends on the components in the system. As stated by the AXI Spec:
"The protocol does not specify the exact use of the QoS identifier. This specification recommends that AxQOS is used as a priority indicator for the associated write or read transaction. A higher value indicates a higher priority transaction."
Some examples of how it could be used are:
- At points of arbitration where a higher QoS request can be given priority
- Reordering within a device
- Reservation of tracking structures to ensure higher priority requests make progress sooner.
It is also possible for devices to ignore the QoS value.