E2.8 QoS Accept signaling

AXI4 introduced two interface signals to indicate the QoS value of a transaction. AMBA 5 introduced two additional interface signals that enable a slave to indicate the minimum QoS value of transactions that it accepts. The QoS_Accept property is used to indicate whether an interface includes these signals: TRUE The interface includes both VARQOSACCEPT and VAWQOSACCEPT signals. FALSE The interface does not include VARQOSACCEPT or VAWQOSACCEPT. If QoS_Accept is not declared, it is considered FALSE. The QoS_Accept extension is applicable to the following interfaces: • AXI5. • ACE5. • ACE5-Lite. • ACE5-LiteDVM.	QoS Accept signaling时slave 用于通知master它希望服务的最小Qos的值，大于等于此Qos的transaction能被及时的处理，小于此值的transaction会被delay。
QoS Accept signaling is intended for slave components that have different resources available for different QoS levels, typically true with memory controllers. The slave can indicate that it only accepts transactions at a certain QoS level or above when the resources available to lower QoS levels are in use.	对于slave，表示此slave内部有资源能处理大于等于QoS Accept的请求，小于QoS Accept的请求可能会被delay。
QoS Accept signaling can be used as an input to a master interface that might have several different transactions to select from. This permits the master interface to only issue transactions that are likely to be accepted, which avoids unnecessary blocking of the interface. By preventing the issue of transactions that might be stalled for a significant period, the interface remains available for the issue of higher priority transactions that might arrive at a later point in time.
It is permitted for a master interface to issue a transaction that is below the QoS level indicated by the VAxQOSACCEPT signal. However, such a transaction might be stalled for a significant time.	对于master，发送小于VAxQOSACCEPT的Qos transaction是允许的，只是此transaction可能会被delay。
It is permitted for a slave interface to accept a transaction that is below the QoS level indicated by the VAxQOSACCEPT signal, but it is expected that the transaction might be subject to a significant delay.	同样，对于slave，接受小于VAxQOSACCEPT 的Qos transaction是允许的，但是此transaction的处理可能会被显著的delay。
While it is acceptable for a slave to delay a transaction that has a lower priority than the QoS Acceptance level, this specification recommends that such a transaction is not delayed indefinitely. There are several reasons for a lower-priority transaction to be issued on the interface, for example:	slave可以接受低于VAxQOSACCEPT 的transaction    和 slave可以delay低优先级的transaction，但是不能无限期的阻塞，原因有一些，例如：
A delay between a change in the QoS Acceptance value and the ability of the component to adapt to that change.	slave切换VAxQOSACCEPT 需要时间。
A requirement to make progress on a transaction that is Head-of-Line Blocking a higher priority transaction.	高优先级的transaction可能被低优先级的transaction阻塞
A requirement to make progress on a transaction for reasons of starvation prevention.	防止饥饿