[AXI4 专题] response signaling AXI4中的响应信号与对应控制

AXI4 响应信号类型

• RRESP and BRESP signal is 2 bit length, which is indicating of
• b00: OKAY response
• b01: EOKAY response
• b10: SLVERR response
• b11: DECERR response

OKAY response

Normal access okay indicates if a normal access has been successful. Can also indicate an exclusive access failure.
• the success of a normal access
• the failure of an exclusive access
• an exclusive access to a slave that does not support exclusive access.

EXOKAY response

indicating that either the read or write portion of an exclusive access has been successful.

SVLERR

which is used in when the access has reached the slave successfully, but the slave wishes to return an error condition to originating master.
• FIFO/buffer overrun or underrun condition
• unsupported transfer size attempted
• write access attempted to read-only location
• timeout condition in the slave
• access attempted to an address where no registers are present
• access attempted to a disabled or powered-down function.

DECERR

decode error is generated typically by an interconnect component to indicate the there is no slave at the transaction address.

在系统中通常比较重要，对于一些非法slave地址，避免出现死锁的情况

读写响应的区别以及与transaction的关系

对于写响应而言，写响应信号是针对整个transaction的，是整个burst过程，不是每个data transfer 的响应。
对于读响应而言，slave可以针对transaction中的transfer给出不同的response，例如在16个burst beat中，slave可能给出15个OKAY一个SLVERR。

error response与tranfer的关系

AXI协议严格要求了，就算发生错误响应，也必须完成规定数目的transfer，也就说即便read transaction中有SLVERR，SLAVE也必须完成规定数目的burst beat，transfer不会因为错误而取消

decode error 与 transfer的关系

当出现decode error的情况下，也必须发出足够的transfer，而decode error通常是找不到对应地址的slave，而完成这些transfer的component通常是发起decode error的组件，也就是interconnect

master需要具备的能力

因此AXI中，master处理支持outstanding的能力，还必须要有支持精确的错误信号响应。master必须支持在later transfer已经已经中的时候，处理早期的错误响应

default slave的作用

An implementation option is to have the default slave also record the details of decode errors for later determination of how the errors occurred. In this way, the default slave can significantly simplify the debugging process.

一个实现选项是让默认从属设备也记录解码错误的细节，以便以后确定错误是如何发生的。通过这种方式，默认从机可以大大简化调试过程。