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TXRSP

Functional Description

The TXRSP module receives requests from both the MainPipe and MSHR modules directed to the TXRSP channel, arbitrates between them, buffers them in a queue, and ultimately sends them to the CHI TXRSP bus channel. Requests from the MainPipe exit are unconditionally accepted, while requests from MSHR may be blocked. Therefore, the TXRSP module needs to apply backpressure to MSHR and implement flow control at the MainPipe entry to ensure that requests on the MainPipe can enter TXRSP without obstruction.

Feature 1: Flow Control at MainPipe Entry

  1. To ensure that requests on the MainPipe can enter TXRSP non-blockingly at s3/s4/s5, when [the number of requests on the MainPipe that may need to enter TXRSP + the number of valid entries in the queue ≥ the total queue capacity], the TXREQ module must apply backpressure at the MainPipe entry, i.e., the s0/s1 stages.
  2. Backpressure at the s1 stage is necessary because snoops received by RXSNP may be processed directly on the MainPipe and then enter the TXRSP channel, requiring backpressure on s1's sinkB requests. Backpressure at the s0 stage is needed because some MSHR tasks must enter the TXRSP channel, and MSHR tasks enter the pipeline at s0, necessitating backpressure on s0's mshrTask. The blocking condition is temporarily denoted as noSpace.

Feature 2: Backpressure Logic for MSHR

  1. The arbitration priority of MainPipe is higher than that of MSHR, so when a request from the MainPipe exit is valid, backpressure must be applied to the MainPipe.
  2. Backpressure must be applied to the MainPipe when noSpace is true for the following reason: in the same cycle that MSHR sends a request, the MainPipe may have no requests competing with MSHR, but there could be requests in the MainPipe at stages s1/s2. The MSHR request might occupy queue slots that would otherwise be available for MainPipe requests, causing insufficient queue slots when MainPipe requests reach stages s3/s4/s5. Therefore, MainPipe requests must also be blocked in this scenario.

Overall Block Diagram

TXRSP